Fix assorted Travis / spec issues

Originally I was just planning to add Ruby 2.1.0 and get Travis running with rbx, but in the process of getting that working I discovered a number of other Travis/spec related issues.  These were:

1. Travis was not failing spec runs - If specs were run but had failures, then Travis was reporting green because the system call being used exited with a return code of 0.  Changing the Rakefile to use the standard spec task fixed this.

2. Fixed a spec that's been failing for all Rubies.  Introduction of the use of the `to_json` method caused the \u2028 line separator replacement to fail, as the `to_json` method was converting the raw unicode bytes to the 7-bit safe representations.  Updated the escaper to look for the escaped 7-bit representations.

3. A thread safety spec that fails on JRuby and rbx - Updated a thread safety spec that fails on all VMs that don't use the GIL.  I'm not entirely sure what the spec was intended to test, but now it properly tests that `gon` in different threads access separate thread-local variables.

I also:

1. Added the Rubinius SL gems to the Gemfile, allowing specs to run on Rubinius

2. Added Ruby 2.1.0 to the .travis.yml

which was my original plan.

Specs run green on all platforms with this PR.