Change version of spock-core and groovy

Hi,
Since groovy 2.4 (current 2.4.0-beta-4) we are able to write Android apps in groovy. However robospock in current 0.5.0 is not compatible with groovy 2.4 due to obsolete version of spock-core used. It causes some problems in testing (ex. generated equals method by @Cannonical breaks tests).

To solve this problem I have changed versions of groovy and spock to newest release version (2.3.8 and 0.7-groovy-2.0). This changes will NOT be backwards compatible with robospock 0.5.0 so I have also changed version to 0.6.0-beta-1.
Other changes are recommended by new Android Studio v1.0.1

It is my first pull request ever and so I don't know how to suggest that my pull request should be merged to new branch.
Karol Kowalski - OrdonTeam