workaround for bug in Gradle 1.8

- FileTree.matching() is broken in Gradle 1.8
  - see http://forums.gradle.org/gradle/topics/is_filetree_matching_broken_in_gradle_1_8
  - see http://issues.gradle.org/browse/GRADLE-2900
- withtout this workaround 'gradle jarAll' fails with:

```
> Could not expand ZIP
> '~/.gradle/caches/.../commons-cli-1.2.jar'
```