Changed 'email-domain' from key to filter

Email domain filtering for companies is currently broken in the Ruby gem.

According to the LinkedIn API doc, 'email-domain' functions as a filter, not a key, for the companies resource. It's entered as this format: companies?<filter> instead of companies/<key> as it currently is in the Ruby gem.

https://developer.linkedin.com/documents/company-lookup-api-and-fields