Update the docs regarding the prerequisites of building Buck.
Update the docs regarding the prerequisites of building Buck.

Summary:
Java 7 isn't even available for download anymore, so there's no need to
specify the version. Also assume we won't be supporting Python 2.6 for
long.

Closes https://github.com/facebook/buck/issues/920

Test Plan: docs/soyweb-local.sh

Reviewed By: aiked

fbshipit-source-id: 0cc9159

zhan-xiong/buck,dsyang/buck,k21/buck,shs96c/buck,clonetwin26/buck,davido/buck,Addepar/buck,marcinkwiatkowski/buck,shs96c/buck,robbertvanginkel/buck,daedric/buck,SeleniumHQ/buck,robbertvanginkel/buck,ilya-klyuchnikov/buck,zpao/buck,LegNeato/buck,robbertvanginkel/buck,SeleniumHQ/buck,robbertvanginkel/buck,justinmuller/buck,marcinkwiatkowski/buck,justinmuller/buck,zhan-xiong/buck,justinmuller/buck,nguyentruongtho/buck,JoelMarcey/buck,grumpyjames/buck,justinmuller/buck,Addepar/buck,darkforestzero/buck,shybovycha/buck,shybovycha/buck,vschs007/buck,Addepar/buck,darkforestzero/buck,sdwilsh/buck,SeleniumHQ/buck,shybovycha/buck,brettwooldridge/buck,marcinkwiatkowski/buck,davido/buck,sdwilsh/buck,rmaz/buck,ilya-klyuchnikov/buck,Addepar/buck,vschs007/buck,k21/buck,robbertvanginkel/buck,davido/buck,romanoid/buck,sdwilsh/buck,grumpyjames/buck,SeleniumHQ/buck,nguyentruongtho/buck,sdwilsh/buck,shs96c/buck,romanoid/buck,clonetwin26/buck,LegNeato/buck,JoelMarcey/buck,kageiit/buck,brettwooldridge/buck,dsyang/buck,sdwilsh/buck,marcinkwiatkowski/buck,rmaz/buck,zhan-xiong/buck,LegNeato/buck,k21/buck,shybovycha/buck,zhan-xiong/buck,vschs007/buck,justinmuller/buck,JoelMarcey/buck,robbertvanginkel/buck,zpao/buck,marcinkwiatkowski/buck,grumpyjames/buck,davido/buck,illicitonion/buck,JoelMarcey/buck,robbertvanginkel/buck,clonetwin26/buck,clonetwin26/buck,daedric/buck,zpao/buck,Addepar/buck,rmaz/buck,darkforestzero/buck,SeleniumHQ/buck,daedric/buck,shs96c/buck,robbertvanginkel/buck,romanoid/buck,illicitonion/buck,rmaz/buck,robbertvanginkel/buck,zhan-xiong/buck,dsyang/buck,dsyang/buck,clonetwin26/buck,sdwilsh/buck,darkforestzero/buck,SeleniumHQ/buck,Addepar/buck,davido/buck,grumpyjames/buck,clonetwin26/buck,romanoid/buck,dsyang/buck,rmaz/buck,robbertvanginkel/buck,romanoid/buck,brettwooldridge/buck,illicitonion/buck,nguyentruongtho/buck,Addepar/buck,justinmuller/buck,shybovycha/buck,davido/buck,darkforestzero/buck,vschs007/buck,zhan-xiong/buck,davido/buck,Addepar/buck,shybovycha/buck,shybovycha/buck,illicitonion/buck,sdwilsh/buck,illicitonion/buck,SeleniumHQ/buck,brettwooldridge/buck,justinmuller/buck,grumpyjames/buck,nguyentruongtho/buck,zhan-xiong/buck,rmaz/buck,shybovycha/buck,shs96c/buck,daedric/buck,zhan-xiong/buck,dsyang/buck,brettwooldridge/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,k21/buck,dsyang/buck,kageiit/buck,marcinkwiatkowski/buck,romanoid/buck,shybovycha/buck,davido/buck,vschs007/buck,sdwilsh/buck,zhan-xiong/buck,grumpyjames/buck,brettwooldridge/buck,illicitonion/buck,sdwilsh/buck,daedric/buck,k21/buck,SeleniumHQ/buck,ilya-klyuchnikov/buck,facebook/buck,darkforestzero/buck,grumpyjames/buck,LegNeato/buck,clonetwin26/buck,JoelMarcey/buck,JoelMarcey/buck,sdwilsh/buck,shs96c/buck,grumpyjames/buck,illicitonion/buck,sdwilsh/buck,ilya-klyuchnikov/buck,justinmuller/buck,illicitonion/buck,brettwooldridge/buck,LegNeato/buck,Addepar/buck,sdwilsh/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,vschs007/buck,k21/buck,dsyang/buck,rmaz/buck,LegNeato/buck,vschs007/buck,JoelMarcey/buck,darkforestzero/buck,ilya-klyuchnikov/buck,zpao/buck,clonetwin26/buck,dsyang/buck,rmaz/buck,ilya-klyuchnikov/buck,vschs007/buck,grumpyjames/buck,clonetwin26/buck,shs96c/buck,dsyang/buck,LegNeato/buck,JoelMarcey/buck,nguyentruongtho/buck,clonetwin26/buck,daedric/buck,darkforestzero/buck,dsyang/buck,SeleniumHQ/buck,sdwilsh/buck,SeleniumHQ/buck,romanoid/buck,SeleniumHQ/buck,facebook/buck,kageiit/buck,clonetwin26/buck,vschs007/buck,nguyentruongtho/buck,justinmuller/buck,robbertvanginkel/buck,k21/buck,SeleniumHQ/buck,brettwooldridge/buck,vschs007/buck,ilya-klyuchnikov/buck,shybovycha/buck,JoelMarcey/buck,shs96c/buck,zhan-xiong/buck,romanoid/buck,rmaz/buck,daedric/buck,k21/buck,davido/buck,shybovycha/buck,illicitonion/buck,davido/buck,justinmuller/buck,vschs007/buck,davido/buck,marcinkwiatkowski/buck,LegNeato/buck,brettwooldridge/buck,brettwooldridge/buck,rmaz/buck,shybovycha/buck,daedric/buck,zhan-xiong/buck,k21/buck,darkforestzero/buck,LegNeato/buck,ilya-klyuchnikov/buck,vschs007/buck,illicitonion/buck,daedric/buck,ilya-klyuchnikov/buck,grumpyjames/buck,rmaz/buck,SeleniumHQ/buck,LegNeato/buck,shs96c/buck,shybovycha/buck,k21/buck,marcinkwiatkowski/buck,darkforestzero/buck,dsyang/buck,kageiit/buck,Addepar/buck,shs96c/buck,illicitonion/buck,justinmuller/buck,rmaz/buck,daedric/buck,JoelMarcey/buck,marcinkwiatkowski/buck,daedric/buck,romanoid/buck,zhan-xiong/buck,facebook/buck,brettwooldridge/buck,romanoid/buck,k21/buck,facebook/buck,LegNeato/buck,zpao/buck,dsyang/buck,marcinkwiatkowski/buck,k21/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,LegNeato/buck,facebook/buck,kageiit/buck,zpao/buck,JoelMarcey/buck,shs96c/buck,JoelMarcey/buck,grumpyjames/buck,justinmuller/buck,clonetwin26/buck,Addepar/buck,romanoid/buck,facebook/buck,vschs007/buck,romanoid/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,robbertvanginkel/buck,daedric/buck,illicitonion/buck,shs96c/buck,facebook/buck,davido/buck,nguyentruongtho/buck,darkforestzero/buck,zpao/buck,darkforestzero/buck,romanoid/buck,davido/buck,shs96c/buck,daedric/buck,rmaz/buck,grumpyjames/buck,darkforestzero/buck,justinmuller/buck,clonetwin26/buck,illicitonion/buck,marcinkwiatkowski/buck,kageiit/buck,Addepar/buck,Addepar/buck,kageiit/buck,LegNeato/buck,brettwooldridge/buck,marcinkwiatkowski/buck,k21/buck