update required php version to 5.6

[andreasschacht]

Hi Steffen,

you are using the array_filter function with the ARRAY_FILTER_USE_KEY-flag in https://github.com/steffenbrem/JsonApiBundle/blob/master/Serializer/JsonApiSerializationVisitor.php#L246 . The flags was introduced in PHP 5.6. So the the required php version should be updated in the composer file.