Update dependency on activerecord and railties to support Rails 4.0.0

I'm trying to update one of my gems (which has a dependency on generator_spec) to support Rails4, but I'm running into an issue because generator_spec is locked to Rails 3.

```
Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    generator_spec (= 0.8.5) ruby depends on
      rails (< 4.0, >= 3.0) ruby

    rails (4.0.0)
```

This pull request updates the gemspec to add Rails 4 support to generator_spec (all the specs pass on Rails 4).