Filters out duplicate offences for Github formatters

When the same exact message is repeated on the same line,
this change removes the duplicates.  The test was updated
to reflect this changed behavior as well.

Hi @mmozuras!  This PR is now in a branch (sorry I didn't read the contributing guidelines before making the first PR against master) and fixes the failing test.

Maybe [#uniq](http://www.ruby-doc.org/core-2.1.4/Array.html#method-i-uniq) could be used here?