pkg-config is required to build Rugged

`ERROR: pkg-config is required to build Rugged.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.`
Merge pull request #26 from moortaube/patch-1

pkg-config is required to build Rugged