Process only <a> tags with href attribute in AhoyEmail::Processor

If the body of an email contains an ```<a>``` tag without a href attribute, we get a NoMethodError in the ```AhoyEmail::Processor#track_links``` method on line 96 in lib/ahoy_email/processor.rb. ```link["href"]``` returns ```nil``` in this case. ```Addressable::URI.parse(nil)``` then throws a NoMethodError because it expects a String.

This method is only for rewriting the href attributes, so all ```<a>``` tags without this attribute can be skipped.