Allow customizations in `run_build` script.

The rspec-rails repo needs to change some of the default functions which
are run. Check for a customization script and source it before running
the standard commands. This allows for repos, such as rspec-rails, to
adjust functions as needed before they are run.

/cc @myronmarston 

Closes rspec/rspec-rails#1140
Won't this always delete files?