Fix parameters to gnupg.GPG()

This code originally used the gnupg Python library [1]. The installation
instructions were recently updated to recommned the use of the python-gnupg
library [2] instead. This code was forked from gnupg, but has a slightly
different interface, so the code was broken after the update.

[1] https://pypi.python.org/pypi/gnupg/1.2.3
[2] https://pypi.python.org/pypi/python-gnupg/0.3.5