Remove SaxMachine as a dependency

[HParker]

Remove SaxMachine as a dependency

[mockdeep]

@HParker this seems like a good idea seeing as how SaxMachine hasn't had a release since 2015. However, one consideration is the fact that SaxMachine supports 3 different XML parsers (nokogiri, ox, and oga). Is there a compelling reason to support these additional parsers, or is it okay to standardize on one?

For what it's worth, Feedjira already has a transitive dependency on Nokogiri via Loofah, and it's likely to be a transitive dependency in most projects, since so many other gems depend on it. Given we already depend on it, it doesn't seem like any existing users will notice much of a difference, though they'd need to update their code if they're configuring it.

[HParker]

Yeah if we drop support for other parsers, i think standardizing on nokogiri is the right choice!

[mockdeep]

Turns out we also depend on Nokogiri directly inside of Preprocessor.