Removing unnecessary require.

This will remove warnings like:
/usr/local/lib/ruby/gems/2.1.0/gems/sshkit-1.3.0/lib/sshkit.rb:3: warning: already initialized constant SSHKit::StandardError
/usr/local/Cellar/ruby/2.1.0/lib/ruby/gems/2.1.0/gems/sshkit-1.3.0/lib/sshkit.rb:3: warning: previous definition of StandardError was here