Fixes LinkedIn endpoints. Renames provider name to simply :linkedin

Hi Décio,

I'm not sure if you're aware but there was a change in the LinkedIn DNS that affected many users of this gem. The LinkedIn OAuth2 client site parameter needs to be updated to "https://api.linkedin.com". This pull request makes that change. See this epic thread on LinkedIn's developer forum: http://developer.linkedin.com/forum/sudden-authentication-failure-production-environment

Also, previous to this endpoint change, I forked this gem to rename the Omniauth provider name to simply `:linkedin`  instead of `:linkedin_oauth2`. This was to provide a cleaner upgrade path for users of the older OAuth 1.0 omniauth adapter for LinkedIn.

If you agree with these changes, I would appreciate your acceptance of the pull request and publishing a new gem. If you need help maintaining/managing the gem, I'd be happy to volunteer to help with that as well if you want to add me as a collaborator on Rubygems.

Thanks,
Nate