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.
Fixed chruby.sh to return immediately if executed from a shell other than bash for zsh.

Without this change, an Ubuntu 12.04 user who installs chruby.sh in /etc/profile.d will not be able to successfully complete a graphical login.  See https://github.com/postmodern/chruby/issues/56