Require tests against rails 6.0, run tests against rails 6.1.alpha

Rails 6.0 requires ruby >= 2.5, so ruby 2.4 is not tested by default
and replaced with 2.6

.travis.yml

@@ -1,27 +1,31 @@
 language: ruby
 cache: bundler
 rvm:
+- 2.6
 - 2.5
-- 2.4
 gemfile:
+- gemfiles/rails_60.gemfile
 - gemfiles/rails_52.gemfile
-- gemfiles/rails_51.gemfile
 matrix:
   include:
   # old rails
+  - rvm: 2.5
+    gemfile: gemfiles/rails_51.gemfile
   - rvm: 2.5
     gemfile: gemfiles/rails_50.gemfile
   - rvm: 2.5
     gemfile: gemfiles/rails_42.gemfile
   # old rubies
+  - rvm: 2.4
+    gemfile: gemfiles/rails_52.gemfile
   - rvm: 2.3
     gemfile: gemfiles/rails_52.gemfile
   # future rails
   - rvm: 2.5
-    gemfile: gemfiles/rails_60.gemfile
-  # Allow rails 6 to fail while master branch is used
+    gemfile: gemfiles/rails_61.gemfile
+  # Allow rails 6.1 to fail while master branch is used
   allow_failures:
   - rvm: 2.5
-    gemfile: gemfiles/rails_60.gemfile
+    gemfile: gemfiles/rails_61.gemfile
 notifications:
   email: false