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
Random idea: we could have our tasks here run a `script/before_update_travis_build` script (if it exists) before starting the build update process, and a `script/after_update_travis_build` script (if it exists) after the process is done (but before committing the code).  This would be done in the directory of the project so it would allow projects to hook in and customize.  For example, for rspec-rails, it could temporarily backup `.travis.yml` to an alternate file name in the before script, then move it it back to `.travis.yml` before committing (but after it gets copied here).

I think that would provide a generalized mechanism for any projects to do before/after customization as needed.