tox.ini: Use pytest-httpbin>=0.0.6
tox.ini: Use pytest-httpbin>=0.0.6

This hopefully fixes SSL timeout errors. 

Fixes #308
zerin108/httpie,jkbrzt/httpie,ardydedase/httpie,alexeikabak/httpie,kevinlondon/httpie,mblayman/httpie,lk1ngaa7/httpie,fritaly/httpie,zerin108/httpie,fritaly/httpie,vietlq/httpie,gnagel/httpie,aredo/httpie,bright-sparks/httpie,alex-bretet/httpie,jakubroztocil/httpie,konopski/httpie,lk1ngaa7/httpie,marklap/httpie,GrimDerp/httpie,sofianhw/httpie,rschmidtz/httpie,a-x-/httpie,bitcoinfees/two1-httpie,codingjoe/httpie,meigrafd/httpie,whitefoxx/httpie,danieldc/httpie,saisai/httpie,paran0ids0ul/httpie,HackerTool/httpie,fontenele/httpie,avtoritet/httpie,paran0ids0ul/httpie,HackerTool/httpie,fontenele/httpie,alex-bretet/httpie,hoatle/httpie,keita1314/httpie,rschmidtz/httpie,saisai/httpie,gnagel/httpie,mblayman/httpie,bitcoinfees/two1-httpie,PKRoma/httpie,keita1314/httpie,ardydedase/httpie,whitefoxx/httpie,aredo/httpie,JPWKU/httpie,rusheel/httpie,MiteshShah/httpie,lingtalfi/httpie,hoatle/httpie,jakubroztocil/httpie,meigrafd/httpie,Batterfii/httpie,vietlq/httpie,PKRoma/httpie,normabuttz/httpie,Bogon/httpie,insionng/httpie,alfcrisci/httpie,JPWKU/httpie,guiquanz/httpie,avtoritet/httpie,lingtalfi/httpie,zerodark/httpie,rgordeev/httpie,Batterfii/httpie,danieldc/httpie,zerodark/httpie,tylertebbs20/Practice-httpie,jcrumb/httpie,jakubroztocil/httpie,codingjoe/httpie,rgordeev/httpie,tylertebbs20/Practice-httpie,normabuttz/httpie,sofianhw/httpie,alfcrisci/httpie,jcrumb/httpie,a-x-/httpie,rusheel/httpie,jkbrzt/httpie,chaos33/httpie,marklap/httpie,jkbrzt/httpie,guiquanz/httpie,bright-sparks/httpie,insionng/httpie,bgarrels/httpie,bgarrels/httpie,MiteshShah/httpie,GrimDerp/httpie,konopski/httpie,kevinlondon/httpie,Bogon/httpie,alexeikabak/httpie