Test against the released version of Rails 5.2

fixes #1985

Author: mikegee

.travis.yml

@@ -141,7 +141,7 @@ matrix:
     - rvm: 2.4.4
       env: RAILS_VERSION='~> 3.1.12'
     - rvm: 2.4.4
-      env: RAILS_VERSION=5.2.0.rc1
+      env: RAILS_VERSION=5.2.0
   allow_failures:
     - rvm: 2.2.2
       env: RAILS_VERSION=master