Use coffee binary of coffee that overrode fork

This solves two potential problems when it comes to forking:

    1) Forking will now work correctly even when `coffee` is not installed
       globally.
    2) Forking when using a locally installed version of `coffee` will fork
       using that version, and not fallback to a globally installed version.

Fixes #2957
Merge pull request #3107 from mal/issue2957

Fork with binary of coffee-script in use, rather than global