Update `_GO_BATS_URL` and `_GO_BATS_VERSION` to bats-core/bats-core

[mbland]

Now that bats-core/bats-core development is progressing apace, and it already incorporates my optimizations (bats-core/bats-core#8), I'll update the _GO_BATS_URL and _GO_BATS_VERSION in lib/bats-main once a bats-core release is tagged.