Make the captured output less noisy

The output from the `run-app` and the `test-app` commands are quite
noisy at the moment (in Grails 2+) because of the funky jline stuff
that's happening. This change adds `-plain-output` to the command
invocations to reduce the clutter.
Merge pull request #9 from pledbrook/patch-2

Make the captured output less noisy