make travis test w/ same Ruby we target on desktop

version 1.8.7 .

As discussed in #2615, the test suite can act differently than the `brew-cask` command if it runs under a different Ruby version.  I'm not 100% certain travis will listen to the `rvm` directive given that the language is `objective-c`.  If not, then `script:` should be changed to contain an appropriate `rvm use` statement.

This patch does not address the issue of running plain `rake test` for developers.