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`).

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
Update .travis.yml to include MRI
Update travis to not permutate across MRI/jdk
Hmm. Travis example doesn't work as advertised.
Change docs to reflect marc4j speedup possibilities
Merge pull request #57 from billdueber/extract_marc4j

Allow code/tests to run under MRI