use/install.rst: use python -m pip instead of pip

Windows pip cannot update itself so Windows instructions need a little fixing and having both formats could cause confusion. Specifying python also can make which pip uses as more clear as pypy doesn't seem to get it's own binary for pip and no binary links there.

Links:
* http://pip.readthedocs.org/en/latest/installing.html#upgrade-pip
    * http://pip.readthedocs.org/en/latest/installing.html#id11
        * https://github.com/pypa/pip/issues/1299