Fixed python_package_upgrade_pip

As far as I know, there is not yet a `pip upgrade` command, only a `pip install --upgrade`.

Previously, calling this function resulted in a fabric abort because pip complains:

    ERROR: unknown command "upgrade"

Tested on pip==1.3.1. Also checked the [latests pip docs (1.4.dev1)](http://www.pip-installer.org/en/latest/usage.html) and there is no `pip upgrade`. Perhaps it was in an older version?