Test with Rails 6.0.0 rc1

Author: robertomiranda

.travis.yml

@@ -11,7 +11,7 @@ env:
   - RAILS_VERSION="~> 5.0.0"
   - RAILS_VERSION="~> 5.1.0"
   - RAILS_VERSION="~> 5.2.0"
-  - RAILS_VERSION="~> 6.0.0.beta1"
+  - RAILS_VERSION="~> 6.0.0.rc1"
 matrix:
   exclude:
     - rvm: 2.4.6