Fix gemspec

Previously installing the gem directory from Github with the `gem 'ruby-hl7', github: 'ruby-hl7/ruby-hl7'` would produce the following error.

```
ruby-hl7 at /Users/caleb/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/bundler/gems/ruby-hl7-39fe76decf7e did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["lib/string.rb"] are not files
```

Looks like the new core_ext files weren't in the gemspec, this pull request fixes the error.