Meta: Remove python-pip as apt dependency
Meta: Remove python-pip as apt dependency

Travis CI automatically setups a virtual environment when the language
is set to python so there's no need for us to try to install pip
separately as it is already installed on Travis CI.

Test plan:
$ ./precommit.sh

Ensure that build passes successfully on Travis CI.

kjedruczyk/phabricator-tools,kjedruczyk/phabricator-tools,bloomberg/phabricator-tools,kjedruczyk/phabricator-tools,aevri/phabricator-tools,cs-shadow/phabricator-tools,bloomberg/phabricator-tools,bloomberg/phabricator-tools,kjedruczyk/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,kjedruczyk/phabricator-tools,aevri/phabricator-tools,cs-shadow/phabricator-tools,cs-shadow/phabricator-tools,bloomberg/phabricator-tools,aevri/phabricator-tools,aevri/phabricator-tools,cs-shadow/phabricator-tools,cs-shadow/phabricator-tools