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.
Ignore local ruby version manager configs.
Special case handling of rspec-rails travis config

rspec-rails is ready to use the shared build config files. The only file
that needs to remain unique is the `.travis.yml` file. Special case the
handling for this file for the one repo.

Extract out logically grouped commands into descriptive method names to
aid in readability. Pass the pathnames all the way through the update
objects, so files are treated more like objects.
Include build hooks for customizations.

To provide more robust customization abilities use before and after
update hooks. This allows a repo, such as rspec-rails, to do any
backups, restoring, or other modification of the generated files.
Merge pull request #78 from rspec/improve-rspec-rails-support

Allow customizations in `run_build` script.