Simplify cache vars and allow for TRANSFORMERS_CACHE env
As it currently stands, "TRANSFORMERS_CACHE" is not an accepted environment variable. It seems that the these variables were not updated when moving from version pytorch_transformers to transformers. In addition, the fallback procedure could be improved. and simplified. Pathlib seems redundant here.

This PR:

- adds the TRANSFORMERS_CACHE environment variable
- change the fall-back order of the cache env variables

File_utils (and perhaps the rest of the library) might benefit from using pathlib more widely. In a future endeavour I might take this on.