Fix running `rake` so that it runs `appraisal:install`

This happens because we are using Rake::Task#execute, which doesn't run
dependencies for the task you are executing. This is mostly useful when
running tests locally -- on Travis this isn't a problem b/c we are
running `rake spec cucumber` instead of just `rake`.

Also, clean up the `appraise` task -- I don't see a reason why we are
spawning a subprocess of `rake` here.