[Neuroimaging] Drop Python 2.6 for nibabel?

[Matthew Brett]

Hi,

Yarik current has a PR into nibabel that needs `collections.Counter`,
available in Python >= 2.7 [1]

https://github.com/nipy/nibabel/pull/437

At the moment, nibabel supports Python 2.6, but Python 2.6 was
released in 2008 [2] isn't maintained any more [3], and numpy is
dropping Python 2.6 in the next release [4].  So, I propose that we
drop Python 2.6 support for nibabel as well.  Any objections?

Cheers,

Matthew

[1] https://github.com/nipy/nibabel/pull/437
[2] https://www.python.org/download/releases/2.6/
[3] https://www.python.org/download/releases/2.6.9/
[4] https://github.com/numpy/numpy/blob/master/doc/release/1.12.0-notes.rst#dropped-support