Use marc-marc4j instead of internal to traject

I've pulled out most of the marc4j<-->ruby-marc code into its own gem, `marc-marc4j` since:

* I want to use it elsewhere,  outside of traject
* I added a ruby-marc to marc4j converter as well
* It probably makes sense for this code to be in its own space

This branch uses that gem, newly uploaded, instead of the traject internal stuff. 
The specfile has been changed approrpriate, and everything is green.