Rails 4 Support

Trying to install this on a rails 4 project, when I run `bundle update` I get

```
Bundler could not find compatible versions for gem "activesupport":
  In snapshot (Gemfile.lock):
    activesupport (4.1.7)

  In Gemfile:
    elastictastic (>= 0) ruby depends on
      activesupport (~> 3.0) ruby
```
