use Warning#relative_path for location path when reporting in codecli…

…mate format

* Since Warning#file could return a reference to the abolute path, and codeclimate
does not support reporting issues in that format, we want to always ensure that
the location of any issue is being reported with a relative location.

lib/brakeman/report/report_codeclimate.rb

@@ -73,7 +73,7 @@ def file_path(warning)
     if tracker.options[:path_prefix]
       (Pathname.new(tracker.options[:path_prefix]) + Pathname.new(warning.file.relative)).to_s
     else
-      warning.file
+      warning.relative_path
     end
   end
 end