Remove duplicate require, dsl does it on its own.

This removes warnings like:

/usr/local/lib/ruby/gems/2.0.0/gems/sshkit-1.3.0/lib/sshkit.rb:3: warning: already initialized constant SSHKit::StandardError
/usr/local/Cellar/ruby/2.0.0-p247/lib/ruby/gems/2.0.0/gems/sshkit-1.3.0/lib/sshkit.rb:3: warning: previous definition of StandardError was here
*