Added lost xdebug version check for 'coverage_enable' option.

[alexanderilyin]

If someone still using old xdebug extension then Code Coverage will not be available for him.

[sebastianbergmann]

PHP_CodeCoverage 1.3 (master branch) requires Xdebug 2.2.

[alexanderilyin]

But why? It' still work on 2.1.2:

PHP 5.3.15 with Suhosin-Patch (cli) (built: Jul 31 2012 14:49:18) 
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
    with Xdebug v2.1.2, Copyright (c) 2002-2011, by Derick Rethans

and versions in generated footer:

Generated by PHP_CodeCoverage 1.3-g0c7e8b4 using PHP 5.3.15 and PHPUnit 3.7.7-1-g994f8c1 at Thu Oct 11 17:09:52 MSK 2012.

[sebastianbergmann]

PHPUnit 3.8 requires PHP 5.4 which is only supported in Xdebug 2.2 (and newer).