Fixed typo - added 's' to guard.run_on_change

I'd been getting this error consistently when using this version of guard-sass with the latest guard-livereload:

> [#] NoMethodError: undefined method `run_on_change' for #<Guard::LiveReload:0x007fa2e3b45310>

Changing this to run_on_changes in sass.rb seems to have fixed it.
Merge pull request #42 from mike-hearn/patch-1

Fixed typo - added 's' to guard.run_on_change