Improve String#squish whitespaces matching

Hi,

My first PR :).
This is a simple improvement of the regex detecting the whitespaces in String#squish. 
The actual regex is an "ASCII version" and my PR replaces it with a more UTF-8 compliant.

More infos on UTF-8 whitespaces : [http://en.wikipedia.org/wiki/Unicode_character_property#Whitespace](http://en.wikipedia.org/wiki/Unicode_character_property#Whitespace)