* freeze pip version, upon installation

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* use PIP_UPDATE environment variable

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* disable shell checking (for now)

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* --disable-pip-version-check

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>