travis: pip -> pip2

travis: pip -> pip2

On travis, for OSX, python2 from homebrew is pre-installed. per [1]:

 python points to the macOS system Python (with no manual PATH modification)
 python2 points to Homebrew’s Python 2.7.x (if installed)
 python3 points to Homebrew’s Python 3.x (if installed)
 pip doesn't exist
 pip2 points to Homebrew’s Python 2.7.x’s pip (if installed)
 pip3 points to Homebrew’s Python 3.x’s pip (if installed)

We will end up using 'python2' for building mesa.

Just use 'pip2' instead of 'pip', as that seems to work for all platforms on
travis.

[1] https://docs.brew.sh/Homebrew-and-Python.html

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>