[bats] update to bats-core community fork

[james-stocks]

BATS is currently at version 0.4 and no maintainers have been available to merge changes in over 3 years.
See sstephenson/bats#150 for discussion on maintainers and forking.

The main fork is now https://github.com/bats-core/bats-core and it's made it to a 1.0 release and is now at v1.1

We should add bats-core as a core plan.
(Though - do we just use the current bats core-plan; or make a new bats-core plan?)

[james-stocks]

Discussed with @smacfarlane that we should update the current bats core-plan to make bats-core the new upstream, with a clear comment to explain the move to the community fork.