Improved the QuestyCaptcha syntax for setting questions and answers

Simple change to one method, QuestyCaptcha::getCaptcha, that allows to
set questions and answers like so:

$wgCaptchaQuestions = array(
	'The capital of England?' => 'London',
	'The capital of France?' => 'Paris',
	'The capital of Spain?' => 'Madrid',
);

Backwards compatibility with the previous syntax has been preserved.

Change-Id: Ife16bfb4c63864f8bc9117dad15136288564e2b0
Merge "Improved the QuestyCaptcha syntax for setting questions and answers"