chruby.sh: Test to make sure GEM_HOME and GEM_ROOT are set before removing their respective bin directories from the path.

Without these tests, it is possible that chruby_reset would end up
removing the /bin directory from the user's path, crippling the shell.

This is just the minimal commit necessary to fix the problem.  It might
be nice to add a corresponding test.
chruby_reset_test.sh: Added a test for the previous commit.

It sets GEM_HOME and GEM_ROOT to be empty.  It sets PATH to something that is guaranteed to have /bin in it, and it makes sure that chruby_reset does not mess up that path.
Merge pull request #141 from DavidEGrayson/pull_request_1

Fix chruby_reset to not remove /bin from the path