added swig to list of dependencies to install in CONTRIBUTING.rst
added swig to list of dependencies to install in CONTRIBUTING.rst

hsduk/lets-encrypt-preview,tyagi-prashant/letsencrypt,g1franc/lets-encrypt-preview,martindale/letsencrypt,jmaurice/letsencrypt,martindale/letsencrypt,deserted/letsencrypt,kevinlondon/letsencrypt,xgin/letsencrypt,PeterMosmans/letsencrypt,goofwear/letsencrypt,digideskio/lets-encrypt-preview,twstrike/le_for_patching,lmcro/letsencrypt,fmarier/letsencrypt,jmhodges/letsencrypt,jsha/letsencrypt,Jadaw1n/letsencrypt,ghyde/letsencrypt,kuba/letsencrypt,sjerdo/letsencrypt,bsmr-misc-forks/letsencrypt,rlustin/letsencrypt,xgin/letsencrypt,dietsche/letsencrypt,hsduk/lets-encrypt-preview,ruo91/letsencrypt,jmhodges/letsencrypt,solidgoldbomb/letsencrypt,g1franc/lets-encrypt-preview,luorenjin/letsencrypt,Bachmann1234/letsencrypt,riseofthetigers/letsencrypt,Sveder/letsencrypt,rutsky/letsencrypt,lbeltrame/letsencrypt,modulexcite/letsencrypt,brentdax/letsencrypt,beermix/letsencrypt,jsha/letsencrypt,bsmr-misc-forks/letsencrypt,sjerdo/letsencrypt,tyagi-prashant/letsencrypt,Hasimir/letsencrypt,VladimirTyrin/letsencrypt,DavidGarciaCat/letsencrypt,deserted/letsencrypt,hlieberman/letsencrypt,armersong/letsencrypt,wteiken/letsencrypt,VladimirTyrin/letsencrypt,mitnk/letsencrypt,skynet/letsencrypt,ahojjati/letsencrypt,modulexcite/letsencrypt,mrb/letsencrypt,kuba/letsencrypt,twstrike/le_for_patching,DavidGarciaCat/letsencrypt,TheBoegl/letsencrypt,jtl999/certbot,Bachmann1234/letsencrypt,goofwear/letsencrypt,mitnk/letsencrypt,diracdeltas/lets-encrypt-preview,dietsche/letsencrypt,letsencrypt/letsencrypt,diracdeltas/lets-encrypt-preview,stewnorriss/letsencrypt,bestwpw/letsencrypt,TheBoegl/letsencrypt,lmcro/letsencrypt,sapics/letsencrypt,armersong/letsencrypt,stweil/letsencrypt,vcavallo/letsencrypt,stweil/letsencrypt,piru/letsencrypt,skynet/letsencrypt,lbeltrame/letsencrypt,PeterMosmans/letsencrypt,tdfischer/lets-encrypt-preview,piru/letsencrypt,rlustin/letsencrypt,ghyde/letsencrypt,Jonadabe/letsencrypt,rutsky/letsencrypt,BillKeenan/lets-encrypt-preview,fmarier/letsencrypt,wteiken/letsencrypt,BillKeenan/lets-encrypt-preview,sapics/letsencrypt,jtl999/certbot,vcavallo/letsencrypt,beermix/letsencrypt,tdfischer/lets-encrypt-preview,Hasimir/letsencrypt,riseofthetigers/letsencrypt,ahojjati/letsencrypt,BKreisel/letsencrypt,rugk/letsencrypt,mrb/letsencrypt,Sveder/letsencrypt,rugk/letsencrypt,brentdax/letsencrypt,BKreisel/letsencrypt,digideskio/lets-encrypt-preview,Jonadabe/letsencrypt,solidgoldbomb/letsencrypt,letsencrypt/letsencrypt,Jadaw1n/letsencrypt,bestwpw/letsencrypt,stewnorriss/letsencrypt,thanatos/lets-encrypt-preview,kevinlondon/letsencrypt,jmaurice/letsencrypt,thanatos/lets-encrypt-preview,ruo91/letsencrypt,hlieberman/letsencrypt,luorenjin/letsencrypt