Dispatch support is load-order dependent

If you load vim-rspec before dispatch, vim-rspec won't know that dispatch is present. The fix is pretty simple -- change the load order, but it might not be obvious to all. In any case, load order dependencies aren't fun to manage. One solution would be to determine the `rspec_command` on first invocation.

Alternatively, we could drop direct support for dispatch from the plugin. In #12, the decision was made to leave it up to the user to set the proper test runner via the `g:rspec_command` configuration variable. Perhaps we should extend this to dispatch support as well?

What are your thoughts?