I had the exact same problem,
And like you i thought it's enough to add the
compile files('libs/signpost-commonshttp4-1.2.1.2.jar')
compile files('libs/signpost-core-1.2.1.2.jar')
It all worked after i did a ./gradlew clean (I'm on a mac, so in your case, the equivalent.)
You should do this command from the Terminal/Command prompt when you're located on the root of your project.
