Removed "then" as a PHP keyword.

There is no reference in the PHP docs as to "then" being a keyword or
reserved word.  This was causing font-locking problems with the Then()
function from Behat.

Signed-off-by: Eric James Michael Ritz <lobbyjones@gmail.com>
Add Michael Dwyer to the list of contributors
Update php-mode-modified for the recent ‘then’ keyword patch