Fixing gemspec / Active Support 4 incompatibility

Fixing gemspec / Active Support 4 incompatibility. This patch prevents Rails 4 RC2 from erring out durring bundle install.

Here's the error that `bundle install` was giving:

```
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    acts_as_commentable_with_threading (>= 0) ruby depends on
      activesupport (~> 3.0) ruby

    rails (= 4.0.0.rc2) ruby depends on
      activesupport (4.0.0.rc2)
```