Switch from Travis CI to CircleCI

Author: bquorning

.circleci/config.yml

@@ -0,0 +1,31 @@
+version: 2.1
+
+steps: &steps
+  steps:
+    - checkout
+    - run: bundle install
+    - run: bundle exec rubocop --parallel
+    - run: bundle exec rspec
+
+jobs:
+  ruby-2-3:
+    docker:
+      - image: circleci/ruby:2.3
+    <<: *steps
+
+  ruby-2-4:
+    docker:
+      - image: circleci/ruby:2.4
+    <<: *steps
+
+  ruby-2-5:
+    docker:
+      - image: circleci/ruby:2.5
+    <<: *steps
+
+workflows:
+  build:
+    jobs:
+      - ruby-2-3
+      - ruby-2-4
+      - ruby-2-5