Update Sundials required version to a numbered release once needed fixes are included in one

[alexander-novo]

Currently, the sundials commit 07d21c2 contains a required fix, but this commit is not included in any numbered release. We've documented that this commit is currently required, but once a new release is pushed, we should update the minimum required version of sundials to be that release.

See #86 (comment).

[pelesh]

Addressed in #160.