Use foreach loop rather than array_map() to invoke do_hook() callbacks

array_map( 'call_user_func', $hooks ) causes segmentation faults on PHP 5.5.12.
Add PHP 5.5 to travis-ci test matrix. See #1208
Merge pull request #1208 from boonebgorges/master

Use foreach loop rather than array_map() to invoke do_hook() callbacks