Updating TargetRubyVersion to 2.4

Given that Ruby 2.3 is past EOL, and 2.4 is approaching EOL, I'm bumping
the targeted version to olded non-EOL ruby version. Note, Hyrax's
gemspec already states that you must use Ruby 2.4 or greater.

This should be a non-breaking change as the following PRs addressed the
Rubocop violations:

* #4250 (and associated SHAs)
  * 12bec9b
  * 571d3d6
  * 9ce198c
* #4251 (and associated SHA)
  * 988a8e3

Author: jeremyf

.rubocop.yml

@@ -4,7 +4,7 @@ inherit_gem:
 inherit_from: .rubocop_fixme.yml
 
 AllCops:
-  TargetRubyVersion: 2.3
+  TargetRubyVersion: 2.4
   DisplayCopNames: true
   Exclude:
     - 'db/**/*'