Added the backward compatibility support on bcrypt verify

Possible solution to the issue #4538. When the user explicit set the backwardCompatibility to true, the verify method of Zend\Crypt\Password\Bcrypt tries to substitute the new $2y$ with $2a$ if PHP < 5.3.7 (and generate a WARNING suggesting to upgrade to PHP 5.3.7+). 
If backwardComaptibility is set to false the verify method throws and Exception with a message to upgrade to PHP 5.3.7+.