Try to fix Travis CI macOS builds

caused by failing bundler install during rvm use.

Also the 2.3 version should be preinstalled.

Author: felixbuenemann

.travis.yml

@@ -53,5 +53,5 @@ matrix:
       env: DB=mariadb10
     - rvm: 2.0.0
       env: DB=mysql57
-    - rvm: 2.0.0
+    - rvm: 2.3
       os: osx