MAINT+TST: update minimum nibabel dependency

Add to travis testing to check in future.
Merge pull request #477 from matthew-brett/update-nibabel-dependency

MAINT+TST: update minimum nibabel dependency
TST: Increase the timeout on the Travis pip install

H/T to @ogrisel for suggesting this:
https://github.com/nipy/dipy/pull/474/files#r20766386
Merge pull request #478 from arokem/travis-time

TST: Increase the timeout on the Travis pip install

H/T to @ogrisel for suggesting this:
https://github.com/nipy/dipy/pull/474/files#r20766386