Match Rubocop TargetRubyVersion with .ruby-version (2.3.3)

Since the application refuses to run on anything other than the version
specified in .ruby-version and the Gemfile, there's no reason to
maintain support for earler versions.

Despite what the [Rubocop docs][] say, rubocop is targeting ruby 2.0
when TargetRubyVersion is unspecified.

[Rubocop docs]: https://github.com/bbatsov/rubocop/blob/master/manual/configuration.md#setting-the-target-ruby-version

.rubocop.yml

@@ -1,7 +1,7 @@
 inherit_from: .rubocop_todo.yml
 
 AllCops:
-  TargetRubyVersion: 2.2
+  TargetRubyVersion: 2.3
   Exclude:
     - 'frontend/**/*'
     - 'vendor/**/*'