Update install.rst
Update install.rst

Former-commit-id: a12a06602ac0ce929857589b83ab1b232b14bd89
danielhers/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,clab/cnn,clab/dynet,clab/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,clab/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet