Less restrictive gem dependencies

Set Sass 3.3 as a _minimum_ requirement, and since Sass 3.3 needs Compass 1.0.0.alpha.18, set that as well. See https://github.com/HugoGiraudel/SassyStrings/blob/master/SassyStrings.gemspec#L34-L35 for an example.
Merge pull request #2 from jedfoster/patch-1

Less restrictive gem dependencies