Remove support for Ruby 2.4

That way we don't have to support old features or jump through hoops to
use Bundler 1.x locally / on Travis.

Author: mcmire

.travis.yml

@@ -18,13 +18,11 @@ gemfile:
   - gemfiles/rails_5_2.gemfile
   - gemfiles/rails_6_0.gemfile
   - gemfiles/no_rails.gemfile
-matrix:
-  exclude:
-    - rvm: 2.4
-      gemfile: gemfiles/rails_6_0.gemfile
 rvm:
   - 2.7
   - 2.6
   - 2.5
-  - 2.4
   #- jruby-9.2.7.0
+branches:
+  only:
+    - master

README.md

@@ -205,7 +205,7 @@ I'll get notified right away!
 ## Compatibility
 
 `super_diff` is [tested][travis] to work with
-Ruby >= 2.4.x,
+Ruby >= 2.5.x,
 RSpec 3.x,
 and Rails >= 5.x.