Allow code/tests to run under MRI

The major change is the complete removal of the
marc4j code (extracted into a separate gem,
`traject_marc4j_reader`) and the guarding of various settings
and tests with checks for whether we're running under JRuby
(via `if defined? JRUBY_VERSION`).

TODO:
  * move traject_marc4j_reader under the traject github organization?
  * move threaded stuff to use concurrent-ruby

## Breaking changes:
* marc4j_reader no longer included
* Default writer changed from Traject::SolrJWriter to
  Traject::JsonWriter

## Non-breaking changes

* solrj_reader only loaded under JRuby
* multi-threaded mode under MRI now generates a
  warning if attempted