Lowering minSDK from 8 to 5.

Running lint shows no dependencies on minSDKVersion 8. Lowering the minSDK to 5 allows bundling this library with a greater number of applications.

Specifically the maintainers of the Fdroid client were hesitant to merge a pull req I wrote to bundle AndroidPinning because their client has a minSDK of 5 versus the AndroidPinning minSDK of 8.

Thanks! Big fan of the library <3