Ruby: auto_link strips emoji

I noticed auto_link strips out emojis in a tweet/text.

Code:
```
#require 'rubygems'
#require 'bundler/setup'
require 'twitter-text'

include Twitter::TwitterText::Autolink

puts 'This is some emoji: 🤔'
puts auto_link('With auto_link: This is some emoji: 🤔')
```

```
$ ruby emoji.rb
This is some emoji: 🤔
With auto_link: This is some emoji:
```