test: Use optimized Bats from mbland/bats

For even greater details, see the comment for the `optimized-20170215`
tag from: https://github.com/mbland/bats

With this commit, on a MacBook Pro with a 2.9GHz Intel Core i5 CPU and
8GB 1867MHz DDR3 RAM, using Bash 4.4.12(1)-release (the latest at the
time of writing), the `./go test` suite now runs in:

  real    1m20.347s
  user    0m55.040s
  sys     0m18.208s

The impact is even more dramatic on Windows. On the same MacBook Pro,
running Windows 10 under VMware Fusion 8.5.3 with 4GB RAM, the timing
for `./go test` before the changes from #156 was in the O(50-60min)
range for every version of Bash I have installed. Afterwards, it was
down in the O(20+min range).

Using Bats with these changes included, running the MSYS2-based Bash
4.3.46(2)-release that ships with (what is probably a slightly outdated
version of) Git for Windows, the suite now runs in:

  real    6m20.981s
  user    1m53.852s
  sys     3m16.015s

Running Bash 4.4.11(2)-release running under Cygwin:

  real    5m22.506s
  user    1m34.462s
  sys     2m41.568s

And running Bash 4.3.11(1)-release as part of the Windows Subsystem for
Linux (i.e. Ubuntu on Windows 10):

  real    3m40.502s
  user    0m36.531s
  sys     3m1.516s

In addition to the Bash versions mentioned above, I've also tested these
changes using the following Bash versions:

- 3.2.57(1)-release: the default for macOS
- 4.2.25(1)-release: the version from the default Ubuntu 12.04.5/Precise
  image on Travis CI
- 4.3.42(1)-release: the latest on Alpine Linux
- 4.3.46(1)-release: the latest on Ubuntu 16.10
- 4.4.12(0)-release: the version from FreeBSD 10.3-RELEASE-p11
- 4.4.12(1)-release: the latest at the time of writing, on both Arch
  Linux and macOS via Homebrew

All of the go-script-bash tests passed on these other platforms in less
than half the time from before.

Author: mbland

lib/bats-main

@@ -53,10 +53,10 @@ export _GO_BATS_DIR="${_GO_BATS_DIR:-$_GO_TEST_DIR/bats}"
 export _GO_BATS_PATH="$_GO_BATS_DIR/libexec/bats"
 
 # Version of Bats to fetch if `_GO_BATS_DIR` is missing
-export _GO_BATS_VERSION="${_GO_BATS_VERSION:-master}"
+export _GO_BATS_VERSION="${_GO_BATS_VERSION:-optimized-20170205}"
 
 # URL of the Bats git repository to clone to `_GO_BATS_DIR`
-export _GO_BATS_URL="${_GO_BATS_URL:-https://github.com/sstephenson/bats.git}"
+export _GO_BATS_URL="${_GO_BATS_URL:-https://github.com/mbland/bats.git}"
 
 # Set this to nonempty if you wish to collect coverage using kcov by default
 export _GO_COLLECT_BATS_COVERAGE="$_GO_COLLECT_BATS_COVERAGE"