Update rubocop inclusion and exclusion directives to be less strict
Update travis config to exclude 1.9.2 from the testing matrix

* Move jruby entries to the matrix to set the JRUBY_OPTS. While this
  is only technically necessary for proper simplecov reporting, the
  devtools gem does serve as a kind of template for ROM style
  gems, so I figured this was a good change to make since it does
  not have any downsides.
Remove json dependency that was only needed for Ruby 1.9.2 support