Make "$forbiddenFunctions" property public in "Generic.PHP.ForbiddenFunctions" sniff sub-classes

In PHP_CodeSniffer 2.x the `$forbiddenFunctions` property of `Generic.PHP.ForbiddenFunctions` sniff was made public so any sub-class of that sniff that still has it with `protected` visibility will trigger following error:

```
PHP Fatal error:  Access level to WordPress_Sniffs_PHP_DiscouragedFunctionsSniff::$forbiddenFunctions must be public (as in class Generic_Sniffs_PHP_ForbiddenFunctionsSniff) in .../CodingStandards/WordPress-Coding-Standards/WordPress/Sniffs/PHP/DiscouragedFunctionsSniff.php on line 26
PHP Stack trace:
PHP   1. {main}() /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/phpunit:0
PHP   2. PHPUnit_TextUI_Command::main($exit = *uninitialized*) /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/phpunit:56
PHP   3. PHPUnit_TextUI_Command->run($argv = array (0 => '/home/alex/.composer/vendor/bin/phpunit', 1 => '--filter', 2 => 'WordPress', 3 => 'tests/AllTests.php'), $exit = TRUE) /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:138
PHP   4. PHPUnit_Runner_BaseTestRunner->getTest($suiteClassName = 'tests/AllTests', $suiteClassFile = '.../PHP_CodeSniffer/tests/AllTests.php', $suffixes = array (0 => 'Test.php', 1 => '.phpt')) /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:160
PHP   5. ReflectionMethod->invoke(NULL, 'PHP_CodeSniffer_AllTests') /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php:124
PHP   6. PHP_CodeSniffer_AllTests::suite('PHP_CodeSniffer_AllTests') /mnt/hd/home/alex/.composer/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php:124
PHP   7. PHP_CodeSniffer_Standards_AllSniffs::suite() .../PHP_CodeSniffer/tests/AllTests.php:75
PHP   8. include_once() .../PHP_CodeSniffer/tests/Standards/AllSniffs.php:114
```