fix Imagick::count() with PHP 5.6.0RC2

With PHP 5.6.0RC2, tests/021-countable.phpt fails:

    TEST 21/27 [tests/021-countable.phpt]
    ========DIFF========
    001+ Warning: Imagick::count() expects exactly 0 parameters, 1 given in /dev/shm/BUILD/php-pecl-imagick-3.2.0/NTS/tests/021-countable.php on line 10
    002+ 0
    001- 3
    ========DONE========
    FAIL Test countable interface [tests/021-countable.phpt] 

Implementing the count_elements handler solves this issue.
You're right, no need for conditional here.
This handler exists at least in 5.0.0

From my understanding, tests
* count($a) => if the handler is defined, it is used, else the count() method is called (with 1 arg in 5.6, which explain this issue as no arg as expected).
* $a->count() => the method is called.

So, this means that  ca019ad OR 1330250 is enough to fix the issue.