Upgrade to Ruby 2.6.3

Restores some of the changes in
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/257, but
still targets Rubocop for 2.3.

.gitlab-ci.yml

@@ -33,6 +33,17 @@ rubocop:
   except:
     - tags
 
+#ruby 2.6
+rspec:ruby2.6:
+  image: ruby:2.6
+  variables:
+    INSTALL_BUNDLER_VERSION: ~> 1.17.3
+  <<: *rspec_definition
+  tags:
+    - ruby
+  except:
+    - tags
+
 #ruby 2.2
 rspec:ruby2.2:
   image: ruby:2.2

.rubocop.yml

 # Exclude some of GitLab files
 AllCops:
+  TargetRubyVersion: 2.3
   Exclude:
     - 'spec/**/*'
     - 'vendor/**/*'

.ruby-version

-2.3.7
+2.6.3

spec/vcr_cassettes/custom-action-ok-not-json.yml

@@ -46,6 +46,6 @@ http_interactions:
       - '1.436040'
     body:
       encoding: UTF-8
-      string: '""'
+      string: '{'
     http_version:
   recorded_at: Fri, 20 Jul 2018 06:18:58 GMT