Update ruby-sshkit to 1.7.1.

## 1.7.1

  * Fix a regression in 1.7.0 that caused command completion messages to be removed from log output. @mattbrictson

## 1.7.0

  * Update Vagrantfile to use multi-provider Hashicorp precise64 box - remove URLs. @townsen
  * Merge host ssh_options and Netssh defaults @townsen
    Previously if host-level ssh_options were defined the Netssh defaults
    were ignored.
  * Merge host ssh_options and Netssh defaults
  * Fixed race condition where output of failed command would be empty. @townsen
    Caused random failures of `test_execute_raises_on_non_zero_exit_status_and_captures_stdout_and_stderr`
    Also fixes output handling in failed commands, and generally buggy output.
  * Remove override of backtrace() and backtrace_locations() from ExecuteError. @townsen
    This interferes with rake default behaviour and creates duplicate stacktraces.
  * Allow running local commands using `on(:local)`
  * Implement the upload! and download! methods for the local backend

(taca)