Fix #796: do not require cmake for building docs (#797)
Fix #796: do not require cmake for building docs (#797)



Former-commit-id: e476a951cbf681b37509bd4e33b748099f7f2847
xunzhang/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,clab/cnn,shuheik/dynet,xunzhang/dynet,danielhers/dynet,clab/cnn,danielhers/dynet