Specify bundler 1.17 for ruby 2.1 and 2.2

.gitlab-ci.yml

 image: "ruby:2.3"
 
+variables:
+  INSTALL_BUNDLER_VERSION: "~> 2.0.1"
+
 before_script:
   - export PATH=~/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/go/bin
-  - gem install --bindir /usr/local/bin bundler
+  - gem install --bindir /usr/local/bin bundler -v "$INSTALL_BUNDLER_VERSION"
   - cp config.yml.example config.yml
   - bundle install
 
@@ -33,6 +36,8 @@ rubocop:
 #ruby 2.2
 rspec:ruby2.2:
   image: ruby:2.2
+  variables:
+    INSTALL_BUNDLER_VERSION: ~> 1.17.3
   <<: *rspec_definition
   tags:
     - ruby
@@ -42,6 +47,8 @@ rspec:ruby2.2:
 #ruby 2.1
 rspec:ruby2.1:
   image: ruby:2.1
+  variables:
+    INSTALL_BUNDLER_VERSION: ~> 1.17.3
   <<: *rspec_definition
   tags:
     - ruby