Force Python dep upgrade w/ pip in postinst script
Force Python dep upgrade w/ pip in postinst script

In testing I noticed that an upgrade from 0.3.8 to 0.3.9 would not upgrade
the versions of existing Python packages installed by pip unless `--upgrade` was
passed.

ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,micahflee/securedrop,heartsucker/securedrop,heartsucker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,ageis/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,ageis/securedrop,ageis/securedrop,micahflee/securedrop,ageis/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop