Stabilize Travis CI for Xcode 6

Travis CI transitioned to Xcode 6.

* Disabled simulator 'chou' tests; the Core Simulator environment is not stable enough for testing.
* Now using default pre-installed ruby 2.0.0 instead of installing ruby 2.1.1 with rvm (broken).
* Dylibs can be created in Xcode 6, but they require code signing.  Dylib tests are still disabled in Travis CI