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
NF: Starting to implement the SFM.
RF: Make sklearn an optional package.
NF: First pass at fitting the thing.
Starting to work on signal prediction.
Abstract out the design matrix construction. Predict.
BF: Make it work with different in/out situations.

On to test that it does something sensible (signal-wise).
TST + BF: More testing, more fixes.
TST: More tests.
Handle errors in the ODF estimation.
When signal is zero, set params to all-zeros, not nans.
TST: update the tests.
NF: Calculate the ODF from the parameters.
RF: Cache for speed.
DOC: Small fixes in documentation.
DOC: Reference
DOC: example of SFM reconstruction + tracking.