Git ignore an .rvmrc file in the root of the project

Now that there is a Gemfile in the Puppet repository, RVM ignores an rvmrc in
the parent directory - because it assumes that the gemfile defines the full
scope of configuration required.

Since the gemfile is actually useless for the purpose, a project level rvmrc
file is needed to "win", and actually put local configuration in place.

Since things like that configuration are really per-developer, we should
ignore the file if present, though, so that we don't pollute everyone's
working space with our personal opinions on configuration.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>