Added missing require

I was trying to add scss-lint to my rake and I got that `CLI` was uninitialized

```$bundle exec rake
rake aborted!
uninitialized constant SCSSLint::RakeTask::CLI
/Users/rsanchez/.gem/ruby/1.9.3/gems/scss-lint-0.29.0/lib/scss_lint/rake_task.rb:31:in `run_task'
/Users/rsanchez/.gem/ruby/1.9.3/gems/scss-lint-0.29.0/lib/scss_lint/rake_task.rb:23:in `block in initialize'
Tasks: TOP => default => scss_lint
(See full trace by running task with --trace)```

so I added it to the scss_lint gem, and it is working for me.
You should add the `require` under https://github.com/causes/scss-lint/blob/master/lib/scss_lint/rake_task.rb#L29 so it doesn't affect load time.

However, it appears that even with that this task doesn't work--it doesn't allow you to specify a list of files to lint. If you're interested in fixing that, I'd be happy to accept a pull request.