Ensure system ruby is used for “RubyMate”

Ruby 2.0 will close non-standard file descriptor when calling Kernel#exec, which means that the file descriptor used for printing the stack trace is unavailable in our child process.

Ruby 1.8.7 does not accept any options to Kernel#exec so it doesn’t seem like we can make the script compatible with both versions of ruby (at least not with simple code).

This should avoid issue #21.

Related thread on mailing list: http://lists.macromates.com/textmate/2013-March/036014.html