Creating a new release

Derived from https://github.com/scikit-learn/scikit-learn/wiki/How-to-make-a-release

1. Create a new branch x.y.z
2. Change the version number in neurodocker/version.py
3. Tag x.y.z branch as x.y.z (use git tag -a)
4. Build wheel
5. Test installation of wheel in fresh python3 docker container
6. Upload wheel file to PyPi with twine