Use squeeze instead of gsub in SquishNormalizer

According to [these benchmarks](https://gist.github.com/samleb/9455926), using `squeeze` is an order of magnitude faster than `gsub` when dealing with squishing/squeezing spaces on almost all Ruby implementations, except on Rubinius where it doesn't make much of a difference.
Here is a patch that replaces `gsub` by `squeeze` in `SquishNormalizer.normalize`, with all tests green :)