This switches from the older, unmaintained version of bats to one that _is_ maintained. In particular, this fork of the community version - bats-core/bats-core - adds junit timing support, which can be used to track down slow tests.
Once the bats-core/bats-core repository merges junit support, we will switch back to upstream.
Note that tests on OS X will still run using a version that _does not_ have junit timing support.
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