Change the OptParser banner to use Rake.application.name

This allows subclasses of `Rake::Application` (such as Capistrano) to identify
themselves correctly when being used with the `--help` (or with incorrect)
flags.

This small change has not mandated any additional test coverage, and has not
broken the tests for me.
Merge pull request #204 from capistrano/master

Change the OptParser banner to use Rake.application.name.