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.
I prefer the `exec(open('nilearn/version.py').read())` solution. It's Python 2 & 3 compat and breaks any possible cyclic import dependencies.

I wanted to do it in sklearn but failed to do so.