fix Imagick::count() with PHP 5.6.0RC2
add the optional mode (not used)
no need for conditional, count_elements exists (at least) since 5.0.0
Merge pull request #35 from remicollet/issue-count

Fixed Imagick::count() to have the same parameters as the countable interface. This fixes a BC break introduced in PHP 5.6.0RC2, which may be reverted or fixed, but respecting the countable interface is the correct behaviour anyway.