Add nibabel dependency to setup.py

Some tweaking to the way `__version__` works was needed because dependencies are checked at `import nilearn` time. That meant that setup.py couldn't run unless the dependencies were installed.
OK I have looked at how it is done in scikit-learn but if I am completely honest it feels more obscure than the proposed solution.

FWIW one of the place I took some inspiration is [here](https://github.com/audreyr/cookiecutter-pypackage/issues/1).