Use relative path to refer to hiredis directory
Use relative path to refer to hiredis directory

This fixes #11.

ferdinandrosario/hiredis-rb,loggator/hiredis-rb,dixiS/hiredis-rb,dixiS/hiredis-rb,redis/hiredis-rb,ferdinandrosario/hiredis-rb,redis/hiredis-rb,loggator/hiredis-rb