Relaxed activerecord dependency

I get the following error during `bundle update` with awesome_nested_set 3.0.0.rc.2 and actionpack-action_caching.

```
Bundler could not find compatible versions for gem "actionpack":
  In Gemfile:
    rails (= 4.0.0) ruby depends on
      actionpack (= 4.0.0) ruby

    actionpack-action_caching (~> 1.0.0) ruby depends on
      actionpack (4.0.1.rc3)
```

Relaxing the dependency of activerecord solved the issue for me.