[Linter] check wether social_media_url is set to the example value

[fabiopelosin]

From CocoaPods/CocoaPods#1927

[dlackty]

Since the podspec template was generated using user's name:

# s.social_media_url   = "http://twitter.com/#{data[:author_name]}"

There's no such "default" value, right?

[orta]

In this case it's coming from https://github.com/CocoaPods/pod-template/blob/master/NAME.podspec

[fabiopelosin]

Now the template has been fixed and the default value is:

https://twitter.com/EXAMPLE

This can be easily implemented by duplicating this logic of the linter and the relative specs.