Use ruby-2.0.0-p353 for chruby tests

Tests are currently configured to run under ruby 1.9.3-p429 using binaries provided by rvm.io.  Unfortunately, there are no binaries provided for this version of ruby for OS X.  There are binaries provided for OS X for 2.0.0-p353, however, as well as on most other platforms.  The few platforms for which there are no binaries for 2.0.0-p353 also seem to lack those for 1.9.3-p429.  Therefore, I would like to request the version used for testing be changed to 2.0.0-p353.

I have run `make test` with this modification on OS X 10.6, and all tests are passing.