Add ruby 2.6.1 testing and temporarily disable rails 6/master

There's no sense making the travis build take longer when rails 6 will
surely fail.

Author: lgebhardt

.travis.yml

@@ -5,11 +5,17 @@ env:
   - "RAILS_VERSION=5.0.7.1"
   - "RAILS_VERSION=5.1.6.1"
   - "RAILS_VERSION=5.2.2"
-  - "RAILS_VERSION=master"
+#  - "RAILS_VERSION=6.0.0.beta1"
+#  - "RAILS_VERSION=master"
 rvm:
   - 2.3.8
   - 2.4.5
   - 2.5.3
+  - 2.6.1
 matrix:
   allow_failures:
     - env: "RAILS_VERSION=master"
+    - env: "RAILS_VERSION=6.0.0.beta1"
+  exclude:
+    - rvm: 2.6.1
+      env: "RAILS_VERSION=4.2.11"

test/test_helper.rb

@@ -7,6 +7,7 @@
 # export RAILS_VERSION=4.2.6; bundle update rails; bundle exec rake test
 # export RAILS_VERSION=5.0.0; bundle update rails; bundle exec rake test
 # export RAILS_VERSION=5.1.0; bundle update rails; bundle exec rake test
+# export RAILS_VERSION=6.0.0.beta1; bundle update rails; bundle exec rake test
 
 # We are no longer having Travis test Rails 4.1.x., but you can try it with:
 # export RAILS_VERSION=4.1.0; bundle update rails; bundle exec rake test