Ensures that non-template files are eex-evaluated

Fixes https://github.com/phoenixframework/phoenix/issues/371

I reproduced the issue by creating a copy of phoenix in a directory named `templates`. The new_test failed when in a directory with that name.

This PR fixes the issue but wasn't sure how to write a test for it that covers this case even when your phoenix is not in a directory that has `templates` in it.