Add CircleCI configuration
Add CircleCI configuration

We want to try out CircleCI and see if it is not only faster, but also more
robust and less painful to use than Travis CI. Being able to use Docker images
should at least provide some stability that we have not previously experienced.

Compared to the Travis CI configuration, we are missing the ruby-head and
jruby-head tests in CircleCI.

panthomakos/rubocop,rrosenblum/rubocop,sue445/rubocop,akihiro17/rubocop,akihiro17/rubocop,tdeo/rubocop,bquorning/rubocop,deivid-rodriguez/rubocop,maxjacobson/rubocop,sue445/rubocop,maxjacobson/rubocop,smakagon/rubocop,bbatsov/rubocop,haziqhafizuddin/rubocop,vergenzt/rubocop,jfelchner/rubocop,sue445/rubocop,deivid-rodriguez/rubocop,bquorning/rubocop,petehamilton/rubocop,mikegee/rubocop,bbatsov/rubocop,jfelchner/rubocop,palkan/rubocop,panthomakos/rubocop,jmks/rubocop,bquorning/rubocop,smakagon/rubocop,tejasbubane/rubocop,vergenzt/rubocop,rrosenblum/rubocop,meganemura/rubocop,palkan/rubocop,rrosenblum/rubocop,petehamilton/rubocop,mikegee/rubocop,tdeo/rubocop,panthomakos/rubocop,haziqhafizuddin/rubocop,palkan/rubocop,meganemura/rubocop,jmks/rubocop,jfelchner/rubocop,jmks/rubocop,deivid-rodriguez/rubocop,maxjacobson/rubocop,petehamilton/rubocop,tdeo/rubocop,meganemura/rubocop,tejasbubane/rubocop,mikegee/rubocop,akihiro17/rubocop,tejasbubane/rubocop,vergenzt/rubocop