Small fixes for FindRapidJson.cmake

This tiny fix cleans up the FindRapidJson.cmake file, which wasn't
actually searching for the rapidjson.h header correctly but instead
assuming a particular location for it, making some of the other code in
the file redundant.

Change-Id: I5fe3c664b8a078b6610440a8ae4173f38d57a84b
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3631
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins