Diffy doesn't work on docker ruby Alpine Linux image

This happens when trying to run Diffy on docker ruby Alpine Linux image:
```
/ # gem install diffy
Fetching: diffy-3.2.0.gem (100%)
Successfully installed diffy-3.2.0
1 gem installed
/ # irb
irb(main):001:0> require 'diffy'
=> true
irb(main):002:0> Diffy::Diff.new("Hello!\n","Hallo!\n").to_s(:text)
=> ""
```

Diffy works well on other Ruby docker images, such as default Ruby.