Support rails 4.0

If I use this gem with rails 4, the error occurs like below:

```
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    mecab-ext (>= 0) ruby depends on
      activesupport (~> 3.2.13) ruby

    rails (= 4.0.0.rc1) ruby depends on
      activesupport (4.0.0.rc1)
```

Rails 4 is still rc1, but I think you don't have to restrict this gem to only rails 3.2.
If you think this request is appropriate, please merge this request.