Use different Gemfiles for testing Rails 4.2 and Rails 5.0.

.gitlab-ci.yml

 image: ruby:2.1
-before_script:
-  - ./scripts/prepare_build.sh
-  - ruby -v
-  - gem install bundler --no-ri --no-rdoc
-  - bundle install -j $(nproc) --path vendor
-
 cache:
   paths:
     - vendor/ruby
-bundler:audit:
+.rails-42: &rails-42
+  before_script:
+    - ./scripts/prepare_build.sh
+    - ruby -v
+    - gem install bundler --no-ri --no-rdoc
+    - bundle install --gemfile=gemfiles/rails42.gemfile -j $(nproc)
+
+.rails-50: &rails-50
+  before_script:
+    - ./scripts/prepare_build.sh
+    - ruby -v
+    - gem install bundler --no-ri --no-rdoc
+    - bundle install --gemfile=gemfiles/rails50.gemfile -j $(nproc)
+
+rspec-ruby21-rails42:
+  <<: *rails-42
   script:
-    - "bundle exec bundle-audit check --update --ignore OSVDB-115941"
-rspec:
+    - bundle exec rspec spec
+rspec-ruby23-rails42:
+  image: ruby:2.3.1
+  <<: *rails-42
   script:
-  - bundle exec rspec spec
-rspec2.3:
+    - bundle exec rspec spec
+rspec-ruby23-rails50:
   image: ruby:2.3.1
+  <<: *rails-50
   script:
-  - bundle exec rspec spec
+    - bundle exec rspec spec
+
+.static-analysis: &static-analysis
+  image: ruby:2.3.1
+  before_script:
+    - ./scripts/prepare_build.sh
+    - ruby -v
+    - gem install bundler --no-ri --no-rdoc
+    - bundle install -j $(nproc)
 rubocop:
+  <<: *static-analysis
   script:
     - bundle exec rubocop
+
+bundler:audit:
+  <<: *static-analysis
+  script:
+    - "bundle exec bundle-audit check --update --ignore OSVDB-115941"

gemfiles/rails42.gemfile

+source "https://rubygems.org"
+
+gemspec :path => "../"
+
+gem 'activesupport', '~> 4.2.0'
+
+group :development do
+  gem 'rubocop'
+  gem 'rubocop-rspec', '1.5.1'
+  gem 'coveralls', require: false
+  gem 'rspec', '~> 3.0'
+  gem 'rspec-mocks'
+  gem 'rspec-its'
+  gem 'webmock'
+  gem 'guard'
+  gem 'guard-rspec'
+  gem 'pry'
+  gem 'rake'
+end
+
+group :test do
+  gem 'simplecov', require: false
+end
+
+group :development, :test do
+  gem 'bundler-audit', '~> 0.5.0', require: false
+end

gemfiles/rails50.gemfile

+source "https://rubygems.org"
+
+gemspec :path => "../"
+
+gem 'activesupport', '~> 5.0.0'
+
+group :development do
+  gem 'rubocop'
+  gem 'rubocop-rspec', '1.5.1'
+  gem 'coveralls', require: false
+  gem 'rspec', '~> 3.0'
+  gem 'rspec-mocks'
+  gem 'rspec-its'
+  gem 'webmock'
+  gem 'guard'
+  gem 'guard-rspec'
+  gem 'pry'
+  gem 'rake'
+end
+
+group :test do
+  gem 'simplecov', require: false
+end
+
+group :development, :test do
+  gem 'bundler-audit', '~> 0.5.0', require: false
+end