Add 6.0 and remove EOL Rails and Ruby

Rails 5.0 and 5.1 as well as Ruby 2.2, 2.3, and 2.4 are EOL. We can
remove them.

Adds 6.0 Gemfile for testing.

.travis.yml

@@ -1,27 +1,16 @@
 cache: bundler
 rvm:
-  - 2.2.10
-  - 2.3.8
-  - 2.4.5
   - 2.5.3
   - 2.6.1
   - ruby-head
 gemfile:
-  - gemfiles/Gemfile-rails-5-0
-  - gemfiles/Gemfile-rails-5-1
   - gemfiles/Gemfile-rails-5-2
+  - gemfiles/Gemfile-rails-6-0
   - gemfiles/Gemfile-rails-master
 matrix:
   allow_failures:
     - rvm: ruby-head
   fast_finish: true
-  exclude:
-    - rvm: 2.2.10
-      gemfile: gemfiles/Gemfile-rails-master
-    - rvm: 2.3.8
-      gemfile: gemfiles/Gemfile-rails-master
-    - rvm: 2.4.5
-      gemfile: gemfiles/Gemfile-rails-master
 env:
   global:
     - "JRUBY_OPTS=-Xcext.enabled=true"