Disable the pip version check message for uninstalls

[jzoldak]

@nedbat @benpatterson
Similar to #9655.

For quieting these messages from the current output:

pip freeze
pip uninstall -y South
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Uninstalling South-1.0.1:
  Successfully uninstalled South-1.0.1
pip uninstall -y edxval
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Uninstalling edxval-0.0.5:
  Successfully uninstalled edxval-0.0.5
pip uninstall -y django-storages
You are using pip version 6.0.8, however version 7.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Uninstalling django-storages-1.1.5:
  Successfully uninstalled django-storages-1.1.5

[benpatterson]

LGTM

[nedbat]

👍