Informative message when checking Cython version fails (#1251)
Informative message when checking Cython version fails (#1251)

Will help with issues such as #548
clab/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,clab/cnn,clab/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,danielhers/dynet,clab/cnn,clab/cnn