Fix middleman-compass version, use same as Middleman

I am trying to use middleman from master but got

```
Bundler could not find compatible versions for gem "middleman-compass":
  In Gemfile:
    middleman (>= 0) ruby depends on
      middleman-compass (~> 4.0.0) ruby

    middleman-compass (4.0.0.pre.0)
```