Add Rails 6 to TravisCI build. (#510)

* Add Rails 6 to build matrix.

* Specify beta for Rails 6.

.travis.yml

@@ -16,10 +16,14 @@ script:
 
 matrix:
   include:
-    # Bleeding edge
+    # Bleeding edge Ruby
     - rvm: ruby-head
       gemfile: test/gemfiles/5.2.gemfile
 
+    # Next version of Rails
+    - rvm: 2.5.0
+      gemfile: test/gemfiles/6.0.gemfile
+
     # Running one job to execute DANGER bot and linting
     - rvm: 2.5.0
       gemfile: test/gemfiles/5.2.gemfile
@@ -30,3 +34,5 @@ matrix:
 
   allow_failures:
     - rvm: ruby-head
+    - rvm: 2.5.0
+      gemfile: test/gemfiles/6.0.gemfile

test/gemfiles/6.0.gemfile

@@ -0,0 +1,17 @@
+source "http://rubygems.org"
+
+gemspec path: "../../"
+
+gem "rails", "~> 6.0.0.beta1"
+
+group :development do
+  gem "rubocop", require: false
+end
+
+group :test do
+  gem "diffy"
+  gem "equivalent-xml"
+  gem "mocha"
+  gem "sqlite3"
+  gem "timecop", "~> 0.7.1"
+end