Run tests in CircleCI

This switches from running the CI pipeline for Coach in CircleCI,
which we generally use across GoCardless, rather than TravisCI.
The configured workflow will test the project on Ruby 2.3, 2.4
and 2.5 using Rails versions 4.2.10 and 5.1.5.

As part of this process, we:

* Start testing against Ruby 2.5.x, which we weren't doing before
* Move to the most recent versions of Rails in 4.x and 5.x, rather
than slightly out of date versions
* Add `rspec_junit_formatter` to the gem's development
dependencies to get nicely-formatted test results in Circle

.circleci/config.yml

@@ -0,0 +1,96 @@
+version: 2
+
+references:
+  steps: &steps
+    - checkout
+
+    - type: shell
+      name: Write RAILS_VERSION to a file so we can use it for caching purposes
+      command: echo "$RAILS_VERSION" > ~/RAILS_VERSION.txt
+
+    - type: cache-restore
+      key: coach-bundler-{{ checksum "coach.gemspec" }}-{{ checksum "~/RAILS_VERSION.txt" }}
+
+    - run: gem install bundler
+
+    - run: bundle install --path vendor/bundle
+
+    - type: cache-save
+      key: coach-bundler-{{ checksum "coach.gemspec" }}-{{ checksum "~/RAILS_VERSION.txt" }}
+      paths:
+        - vendor/bundle
+
+    - type: shell
+      command: |
+        bundle exec rspec --profile 10 \
+                          --format RspecJunitFormatter \
+                          --out /tmp/test-results/rspec.xml \
+                          --format progress \
+                          spec
+
+    - type: store_test_results
+      path: /tmp/test-results
+
+    - run: bundle exec rubocop
+jobs:
+  build-ruby22-rails515:
+    docker:
+      - image: ruby:2.2
+        environment:
+          - RAILS_VERSION=5.1.5
+    steps: *steps
+  build-ruby22-rails4210:
+    docker:
+      - image: ruby:2.2
+        environment:
+          - RAILS_VERSION=4.2.10
+    steps: *steps
+  build-ruby23-rails515:
+    docker:
+      - image: ruby:2.3
+        environment:
+          - RAILS_VERSION=5.1.5
+    steps: *steps
+  build-ruby23-rails4210:
+    docker:
+      - image: ruby:2.3
+        environment:
+          - RAILS_VERSION=4.2.10
+    steps: *steps
+  build-ruby24-rails515:
+    docker:
+      - image: ruby:2.4
+        environment:
+          - RAILS_VERSION=5.1.5
+    steps: *steps
+  build-ruby24-rails4210:
+    docker:
+      - image: ruby:2.4
+        environment:
+          - RAILS_VERSION=4.2.10
+    steps: *steps
+  build-ruby25-rails515:
+    docker:
+      - image: ruby:2.5
+        environment:
+          - RAILS_VERSION=5.1.5
+    steps: *steps
+  build-ruby25-rails4210:
+    docker:
+      - image: ruby:2.5
+        environment:
+          - RAILS_VERSION=4.2.10
+    steps: *steps
+
+workflows:
+  version: 2
+  tests:
+    jobs:
+      - build-ruby22-rails515
+      - build-ruby22-rails4210
+      - build-ruby23-rails515
+      - build-ruby23-rails4210
+      - build-ruby24-rails515
+      - build-ruby24-rails4210
+      - build-ruby25-rails515
+      - build-ruby25-rails4210

coach.gemspec

@@ -22,5 +22,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "pry", "~> 0.10"
   spec.add_development_dependency "rspec", "~> 3.2"
   spec.add_development_dependency "rspec-its", "~> 1.2"
+  spec.add_development_dependency "rspec_junit_formatter", "~> 0.3.0"
   spec.add_development_dependency "rubocop", "~> 0.52.0"
 end