Code generation depends on hard-coded fs path

[bzz]

Right now go generate always assumes Linguist version present in .linguist dir in FS.

As discussed https://github.com/src-d/enry/pull/189#discussion_r252043960, it would be nice to unify it with all the test and rely in both places on single on LINGUIST_REPO env var, instead of ENRY_TEST_REPO in tests and hard-code in generator.