Update or create bats formula on homebrew-core

Currently, `brew info bats` would print this:

```shell
$ brew info bats
bats: stable 0.4.0, HEAD
TAP-compliant test framework for Bash scripts
https://github.com/sstephenson/bats
Not installed
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/bats.rb
```

This needs to be updated in Homebrew's core tap, or a new one should be created.