tests: switch to bats-core

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.

tests/unit/10_ps-dockerfile.bats

@@ -12,7 +12,7 @@ teardown() {
   global_teardown
 }
 
-# @test "(ps) dockerfile" {
+# @test "(ps) dockerfile base" {
 #   # CI support: 'Ah. I just spoke with our Docker expert --
 #   # looks like docker exec is built to work with docker-under-libcontainer,
 #   # but we're using docker-under-lxc. I don't have an estimated time for the fix, sorry