Fix removal of "v" characters from DOMText

`\t\r\n\f` are the standard whitespace characters specified.  `\v` matches a
vertical tab but support was only added in PHP 5.2.4.
(http://php.net/manual/en/regexp.reference.escape.php).  Running the test suite
on a simple string that contained a "v" character would result in all `v` characters
being stripped out of the result. (See http://cl.ly/image/3D143V0E2N0x)

Because of this oddity, I simply removed the \v.  This might not be the desired
solution, but I figured i'd open a pull.  It fixed a number of failing test cases.

I'm running on PHP 5.5.9RC1

http://cl.ly/image/470c2U1X3e0a