Update definitions.txt

This update of the `definitions.txt` file was motivated by an issue we're currently experiencing in [GitHub Pages](https://pages.github.com/), where this gem is used in relation to setting up custom domains for GitHub Pages sites.

The specific issue which was reported and fixed in the Public Suffix List here:

https://bugzilla.mozilla.org/show_bug.cgi?id=1051875

I just used the rake task included to update the file:

```
bundle exec rake upddef
```

When running the tests locally, I was receiving exactly the same list of errors which are shown for the Travis CI build for the last version of the library you released:

https://travis-ci.org/weppos/publicsuffix-ruby/builds/27782654

So, I'm not sure what to do about that, however it would definitely be nice to have the tests passing for future contributors.

We'd love to have a new version of the gem released if this PR is merged. :heart: