Add rails-6 to build matrix

.travis.yml

@@ -1,14 +1,28 @@
 language: ruby
 cache: bundler
 rvm:
-  - 2.5
-  - 2.4
-  - 2.3
+- 2.5
+- 2.4
 gemfile:
-  - gemfiles/rails_52.gemfile
-  - gemfiles/rails_51.gemfile
-  - gemfiles/rails_50.gemfile
-  - gemfiles/rails_42.gemfile
+- gemfiles/rails_52.gemfile
+- gemfiles/rails_51.gemfile
+matrix:
+  include:
+  # old rails
+  - rvm: 2.5
+    gemfile: gemfiles/rails_50.gemfile
+  - rvm: 2.5
+    gemfile: gemfiles/rails_42.gemfile
+  # old rubies
+  - 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
+  allow_failures:
+  - rvm: 2.5
+    gemfile: gemfiles/rails_60.gemfile
 notifications:
   email: false