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

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

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##### A Note About Tests

No additional tests have been added for this PR, however existing cluster tests continue to pass, and no longer throw `execvp(): No such file or directory` / `Error: spawn ENOENT` errors when coffee-script is not installed globally.