version to 2.1.2

* some minor cleanups to the gem meta (4451e18e0a64fda36853973279075d7ce5cef193)
* some general cleanups and modernizations (#16)
* have `RunError` *optionally* take a custom backtrace (#17)
* read output from stdout and stderr while cmd is running (#18)
* Add a bench mark script to help measure performance impact of changes (#20)
* use a "self-pipe" to handle cmd timeouts (#21)
* Some minor tweaks to the setup (#22)

/cc @jcredding