Support Rails 4 as default.

detect_rails_version now detect 4.0.0 as default. Before it will detect 3.2.14. This will cause errors like:

```
Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    activeadmin (~> 0.6.1) ruby depends on
      rails (< 4, >= 3.0.0) ruby

    rails (4.0.0)
```
after a freshly created Rails 4 app.

Steps to reproduce:

(Rails 4.0.0)
1. `$ rails new foo --skip-bundle`
2. Edit Gemfile, add `gem 'activeadmin', '~> 0.6.1'`
3. bundle install