Fix all uncompressed artifacts stored as just '.tar'

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38738752

Reviewed at https://rbcommons.com/s/twitter/r/1197/

src/python/pants/cache/local_artifact_cache.py

@@ -79,4 +79,4 @@ def _cache_file_for_key(self, cache_key):
     # Note: it's important to use the id as well as the hash, because two different targets
     # may have the same hash if both have no sources, but we may still want to differentiate them.
     return os.path.join(self._cache_root, cache_key.id, cache_key.hash) + \
-           '.tar.gz' if self._compress else '.tar'
+           ('.tar.gz' if self._compress else '.tar')