Summary:
We had to go back and update the g++ path for 4.4.fb-4.8.fb. So the
path is now fixed on the branches, but can't be fixed on the tags since they're
immutable. By making format compatibility tests use branch names (when
available), backported fixes like this will be used without having to re-release.

Also removed v1.5.7 and v2.1 because make fails.

Test Plan:
  $ build_tools/rocksdb-lego-determinator run_format_compatible

Reviewers: sdong, lightmark, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D59355