Merge branch 'feature/improve-ci' into 'master'

GitLab CI support

See merge request !1

.gitlab-ci.yml

+.specs: &specs
+  cache:
+    paths:
+      - vendor/ruby
+  before_script:
+    # install dependent gems:
+    - bundle install
+  script:
+    - bundle exec rake spec
+
+ruby-187:
+  image: "jonwood/ruby-1.8"
+  allow_failure: true
+  <<: *specs
+  before_script:
+    - gem install bundler
+    - apt-get update && apt-get install git-core build-essential -y
+    - bundle install
+ruby-193:
+  image: "zedtux/ruby-1.9.3"
+  allow_failure: true
+  <<: *specs
+  before_script:
+    - gem install bundler
+    - apt-get update && apt-get install git-core build-essential -y
+    - bundle install
+ruby-20:
+  image: ruby:2.0
+  allow_failure: true
+  <<: *specs
+ruby-21:
+  image: ruby:2.1
+  <<: *specs
+ruby-22:
+  image: ruby:2.2
+  <<: *specs
+ruby-23:
+  image: ruby:2.3
+  <<: *specs