fixed building with gradle without signing

This fixes the error during a gradle build reported in https://github.com/evilsocket/dsploit/issues/717, so potential contributors should be able to build the dsploit.apk as soon as they have all necessary android packages.
2c7fcd772b540fa0fd36d7add856b9e6e0fb1dcf fixed it "only a little bit" ;)
As I understand the keystore file does not need to be copied to dSploit/ anymore, so the build instructions on dsploit.net still need a bit of an update though.