Fix travis ruby config and limit to supported SDKs

Specifying a 2.0 or 1.9 ruby version seems to yield a broken RVM
environment on Travis CI right now, but 2.1.1 works.  The previous
commit worked around this, but it felt like more of a hack.

Running on SDK 7.1 is also removed since iOSSpecs launched from the
command line is currently broken.

See: http://blog.travis-ci.com/2014-04-15-xcode-51-ios-71-osx-109/
Merge pull request #221 from pivotal/fix-travis-build

Fix travis ruby config and limit to supported SDKs