Replace "nastevens" with "assert-rs" for repo move

[nastevens]

The repo has been relocated from "nastevens" github location to "assert-rs" - at a minimum the appveyor and travisci links will need to be updated, there may be other places.

[epage]

For appveyor, you can find it at
https://ci.appveyor.com/project/epage/predicates-rs