Use `python -m pip` to ensure updated pip is used

/usr/bin/pip is usually an OS level wrapper for pip. Calling
`python -m pip` ensures that we are getting the pip we are
installing rather than the OS local version.

Change-Id: I054d1cc0455c700e00450295cf76d925cff99f73
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>