pip: pulling updates from remote git repository
pip searches for the library in the Python package index. Your version is newer than the newest one in there, so pip won't update it.
You'll have to reinstall from Git:
$ pip install git+git://github.com/scikit-learn/scikit-learn@main
You need to install the version from github, or locally.
The way I usually do is that I git clone the repository locally and I run python setup.py install or python setup.py develop on it so I'm sure about the version being used.
Re-issuing the command you've done the first time with the upgrade flag would do the trick otherwise.:
pip install --upgrade git+git://github.com/scikit-learn/scikit-learn@main
IIRC, Pip installs based on pypi. If you want to upgrade to the version that is currently hosted on github, then you are going to have to use the url from github.