Fix travis.yml jruby configuration

Author: Sam Phippen

.travis.yml

@@ -50,7 +50,7 @@ matrix:
     # Rails 5.2 builds >= 2.2.2
     - rvm: jruby-head
       env:
-        - RAILS_VERSION=6-0-stable
+        - RAILS_VERSION='~> 5.2.0'
         - JRUBY_OPT=--dev
     - rvm: 2.6.3
       env: RAILS_VERSION='~> 5.2.0'