Enable Multi-OS feature to run tests on OS X workers

This PR adds ability to run the test suites on OS X (10.9.4) on Travis CI.

The multi-OS feature has been enabled on ruby/ruby.

A few things to note:

1. A `before_install` task (`diff`) is allowed to fail. It is not clear why `make reconfig` creates a different configuration, or how this should be addressed on OS X.
1. Uses OpenSSL installed by Homebrew.
1. On OS X, parallel makes for `encs` and `exts` targets are disabled. Seems to present some race conditions frequently.
1. A test is modified to allow it to pass on OS X. The existing threshold seems a bit too stringent.