Fix for issue https://github.com/christopherperry/RoboBillingLibrary/issues/3

Merged the fix for the AndroidBillingLibrary https://github.com/hpique/AndroidBillingLibrary/commit/a62593764172d0fd650b1dca1d15c59e79aecd49
to this library.

This fixes issue https://github.com/christopherperry/RoboBillingLibrary/issues/3