Update travis CI with rails6 gemfile

Rails 6 is currently at an RC1. This should allow any beta or release candidate version without needing to update it.

The jruby build fails as it can't find a compatible rails 6 release. Since this installs jruby 9.1, perhaps rails 6 only supports jruby 9.2 (latest)? https://travis-ci.org/sass/sassc-rails/jobs/554811782

Author: javierjulio

.travis.yml

@@ -15,6 +15,7 @@ gemfile:
   - gemfiles/sprockets_4_0.gemfile
   - gemfiles/rails_4_2.gemfile
   - gemfiles/rails_5_2.gemfile
+  - gemfiles/rails_6_0.gemfile
 
 rvm:
   - 2.4.6
@@ -26,6 +27,11 @@ matrix:
   allow_failures:
     - gemfile: gemfiles/rails_4_2.gemfile
       rvm: jruby
+    - gemfile: gemfiles/rails_6_0.gemfile
+      rvm: jruby
+  exclude:
+    - gemfile: gemfiles/rails_6_0.gemfile
+      rvm: 2.4.6
 
 notifications:
   email: false

gemfiles/rails_6_0.gemfile

@@ -0,0 +1,6 @@
+source 'https://rubygems.org'
+
+gem "rails", "~> 6.0.a"
+
+# Specify your gem's dependencies in sassc-rails.gemspec
+gemspec path: "../"