Revert "Add back Reek"

This reverts commit 4ce3c50ed82f0088997c649f1308d2a18ec1df05 as it was
causing test failures on Ruby 1.8.7.

See https://travis-ci.org/dkubb/equalizer/jobs/15967485.
Remove non-existant JRuby modes
Add temporary workaround for failing build on Ruby 1.8.7

Rubygems broke builds on Ruby 1.8.7:

https://github.com/travis-ci/travis-ci/issues/1793
https://github.com/rubygems/rubygems/pull/763

This patch is a temporary workaround until rubygems is updated.
Add support for Ruby 2.1.0