Add outputs.upToDateWhen

Especially in this example, seeing no output when running tests a second time is utterly mind-boggling. I use this project as a base for many applications, I change the code under test regularly, but usually don't touch the test-suite. I'd consider a test-suite never ever up to date since you don't know what it's testing.
Merge branch 'patch-1' of git://github.com/antony/geb-example-gradle