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

eunomie/chgo,rstacruz/chruby-fish,eunomie/chgo,ilikepi/chruby,rstacruz/chruby-fish,postmodern/chruby,regularfry/chruby,ilikepi/chruby,regularfry/chruby,JeanMertz/chruby-fish,JeanMertz/chruby-fish,ngpestelos/chruby,kbrock/chruby,kbrock/chruby,ngpestelos/chruby,postmodern/chruby