core_ext/string LoadError

I'm getting this error below when I run `./convertor.rb`:

```
/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:127:in `require': cannot load such file -- core_ext/string (LoadError)
```
Ruby version:
```
$ ruby -v
ruby 2.3.1p112 (2016-04-26)
```

I'm not a Ruby dev, so this is probably a really simple issue.