Remove duplicate configuration for jruby

* Only use the jruby sections where I can turn on the debug
  flag to get proper coverage reports back.
Remove unnecessary --debug flag from jruby-18mode

* The simplecov coverage will not execute under 1.8 so there is no
  point in having this configured.
* Keep jruby-18mode in the matrix/include list so that it will be
  grouped with the other jruby results.