Try to fix Travis CI rbx-2 missing binary error

Author: felixbuenemann

.travis.yml

@@ -55,7 +55,7 @@ matrix:
       env: DB=mariadb10
     - rvm: 2.0.0
       env: DB=mysql57
-    - rvm: rbx-2
+    - rvm: rbx-2.2.7
       env: RBXOPT=-Xgc.honor_start=true
     - rvm: 2.0.0
       os: osx