Update ANTs to 2.2.0

[chrisgorgo]

Relevant changes:

• different masking in N4BiasFieldCorrection: ANTsX/ANTs@6816c5a
• different metric in brain extraction: ANTsX/ANTs@98644cd
• different registration init in brain extraction: ANTsX/ANTs@10d8c4c

[effigies]

This would be good for #544, since I was testing based on 2.2.0. That said, the way we're using antsRegistration shouldn't be massively different between the two versions. The main thing is we'll need to switch from the old style of specifying masks to new.

Related: nipy/nipype#2078

[chrisgorgo]

I think there are binaries available now (from a third party but still) - no objections to upgrade from me.

[effigies]

Yeah, I've been working with this one.

[effigies]

Looking at these changes, it looks like what we want to do is to make sure the outputs are reasonable; there aren't really interface changes to make. Possibly we could generate a mask before N4BiasFieldCorrection (e.g., with BET using a very liberal threshold), but with brain extraction, as long as it continues to work well, there doesn't seem to be anything to do there...