Move the path 'fixing' script from the custom plugins directory to the lib plugins directory in bash_it

Fixes some issues when the bash_it rvm plugin is enabled.

- The old placement of the script is now causing problems with the
  bash_it rvm plugin, as it gets loaded after the rvm plugin is loaded.
- bash_it/lib gets loaded before bash_it/custom so the PATH gets set
  before any plugins are loaded by bash_it. As such the PATH is set
  correctly at the time the rvm plugin gets loaded.
- in newer versions of rvm we get a constant warning that the .rvm/gems
  bin directory is not the first item in the $PATH. Since homebrew
  doesn't mind being second best to rvm we had to change/move the
  'usr/local/bin' path to come after the rvm bin.
- this is one of the ways we found best to actually make this happen, as
  the script itself can reside in bash_it's lib directory rather than the
  custom plugins directory.