External dependencies are added to the generated IDEA project files.

This is a bugfix. Currently `gradle idea` does not add dependencies that are declared in such format to the .iml file:

    dependencies {
        merged group: 'org.osflash', name: 'as3-signals', version: '0.8', ext: 'swc'
        test group: 'org.flexunit', name: 'flexunit-as3', version: '4.2.0', ext: 'swc'
    }


This commit fixes this issue