add travis_retry param to rvm install

.travis.yml

       - tree
 
 before_install:
-  - rvm install 2.2.5
+  - travis_retry rvm install 2.2.5
   - rvm use 2.2.5
 
 # https://docs.travis-ci.com/user/customizing-the-build#Skipping-the-Installation-Step