Test the 5.2.0 release candidate in place of the beta

Author: wbreeze

.travis.yml

@@ -136,7 +136,7 @@ matrix:
     - rvm: 2.4.2
       env: RAILS_VERSION='~> 3.1.12'
     - rvm: 2.4.2
-      env: RAILS_VERSION=5.2.0.beta2
+      env: RAILS_VERSION=5.2.0.rc1
   allow_failures:
     - rvm: 2.2.2
       env: RAILS_VERSION=master