Update README.md (#1555)

Update the bats link to https://github.com/bats-core/bats-core (which is what pyenv is actually using on [Travis](https://github.com/pyenv/pyenv/blob/master/.travis.yml#L35))

README.md

@@ -379,7 +379,7 @@ The pyenv source code is [hosted on
 GitHub](https://github.com/pyenv/pyenv).  It's clean, modular,
 and easy to understand, even if you're not a shell hacker.
 
-Tests are executed using [Bats](https://github.com/sstephenson/bats):
+Tests are executed using [Bats](https://github.com/bats-core/bats-core):
 
     $ bats test
     $ bats/test/<file>.bats