added explicit  to project. Set is not implicitly loaded on all systems.
Merge remote-tracking branch 'upstream/master' into develop
added ruby 2.1 to travis
whitespace bump to trigger build
added 2.0 and 2.1.5 to travis
added explicit `include set` to project. Set is not implicitly loaded on all systems.

Also added additional ruby versions to travis.

Summary:
Merge remote-tracking branch 'upstream/master' into develop

added ruby 2.1 to travis

whitespace bump to trigger build

added 2.0 and 2.1.5 to travis

Test Plan: run the automated tests. Optionally, use https://lint.travis-ci.org/ to verify the validity of the .travis.yml file.

Reviewers: oliver

Reviewed By: oliver

Differential Revision: http://phab.signifyd.com/D559