Install fail on Ubuntu, sudo not needed for pip

[Whytehorse]

The install instructions for openbazaar server on Ubuntu say to run:
sudo -H pip install -r requirements.txt
This gives the following error:
pkg_resources.DistributionNotFound: The 'pip==9.0.1' distribution was not found and is required by the application

Running without sudo -H fixes this but requires --user as follows:
pip install -r requirements.txt --user