BLACKLIGHT_VERSION not used by travis
Descriptive summary
The BLACKLIGHT_VERSION environment variable isn't being respected by travis in the bundle install.
Rationale
The BLACKLIGHT_VERSION environment variable allows us to test with a specific version of blacklight to ensure compatibility with specific versions especially older versions.
Expected behavior
Travis bundle installs the version of blacklight specified by BLACKLIGHT_VERSION.
Actual behavior
Travis bundle installs the latest blacklight version that matches the gemspec restrictions.
We used to have https://github.com/samvera/hydra-head/commit/5991392d27f8a3765fab54061f3661cc1817f383#diff-93cb22574ece26fa7717ab22d8332c69R6, and I think that was removed in https://github.com/samvera/hydra-head/commit/5944ba518851e2d8aca2354cde7b0085de6e1d50