sprockets requires ruby >= 1.9.3, so just use String#bytesize

We don't need to depend on rack.  The [gemspec requires a ruby version >= 1.9.3](https://github.com/sstephenson/sprockets/blob/cfebc24c30ff447d4b82710f75336c54b8f5a2df/sprockets.gemspec#L39) so there is no reason to depend on [rack's implementation of bytesize](https://github.com/rack/rack/blob/df1506b0825a096514fcb3821563bf9e8fd52743/lib/rack/utils.rb#L331-L340).