Fixes for MacRuby

With these two changes, the rake-pipeline test suite passes on MacRuby. I have not built a site on top of it using MacRuby yet, so I may have more as I go, we'll see. I had to make two changes:


In `rake-pipeline.rb` we are creating this method by aliasing `input_files`. Then we are overriding this method in `rake-pipeline/matcher.rb`. However, because of when we're requiring `rake-pipeline/matcher.rb` the override is happening __before__ the alias. On MacRuby, when the alias runs it clobbers the override causing all input files to match, not just the ones matching the glob in `Assetfile`. For an extremely simplified example see https://gist.github.com/3728490.



SimpleCov is not supported by MacRuby. I pulled this snippet from https://github.com/intridea/multi_json/blob/master/spec/helper.rb to not include SimpleCov on that platform.