get working with circleci

[tansaku]

now that @mihael has demonstrated that we definitely work with TravisCI would be great to work with circleCI as well. Not sure if it makes sense to also enable circleCI on this repo, but was thinking about migrating Localsupport to circleCI to help with deployment automation ...

[mihael]

I recently thought I'd use this gem, but ended up not to, due to CircleCI delivering some nice new features recently. All one has to do is configure test reporting (minitest-ci) and we get timing stats and flaky errors tracking. It won't create a github issue for a flaky test though ~ so that would still be nice to have.