Use Redis to cache Sass files in development

As suggested by @briangonzalez on Meta:
https://meta.discourse.org/t/development-mode-super-slow/2179/51?u=riking

EDIT: I'm getting a NameError with this:

```
[vagrant@precise32:/vagrant (list-setting)]$ bundle exec rails c
/vagrant/config/environments/development.rb:55:in `block in <top (required)>': uninitialized constant ActiveSupport::Cache::RedisStore (NameError)
```