Log Message:
Update ruby-airbrussh to 1.12.0.

## [1.12.0][] (2017-02-10)

### Breaking changes

  * None

### New features

  * Add `SSHKit.config.default_runner_config` option that allows overriding \ 
default runner configs.

## [1.11.5][] (2016-12-16)

### Bug fixes

  * Do not prefix `exec` command
    [PR #378](https://github.com/capistrano/sshkit/pull/378) @dreyks

## [1.11.4][] (2016-11-02)

  * Use string interpolation for environment variables to avoid escaping issues
    with sprintf
    [PR #280](https://github.com/capistrano/sshkit/pull/280)
    @Sinjo - Chris Sinjakli