announce plugin: Don't run AppleScript if headless

As discussed in [gradle-dev](http://gradle.1045684.n5.nabble.com/Announce-Plugin-amp-java-awt-headless-td5712593.html), running AppleScript with `java.awt.headless=true` causes build errors when using the announce plugin. This pull request makes us fall back to `GrowlNotifyBackedAnnouncer` when we're in headless mode and the `growl` announcer is requested.

I've just signed and sent the CLA. Let me know if I can do anything else to help! :)