holder: fix when repositionning/moving member on same day

* if holder has not the index '0', validator fails. Use the method
Collection::first
* change constraint to allow a move on household on same day

src/Bundle/ChillPersonBundle/Validator/Constraints/Household/MaxHolderValidator.php

@@ -29,7 +29,7 @@ class MaxHolderValidator extends ConstraintValidator
 
         $covers = new DateRangeCovering(
             self::MAX_HOLDERS,
-            $holders[0]->getStartDate()->getTimezone()
+            $holders->first()->getStartDate()->getTimezone()
         );
 
         foreach ($holders as $key => $member) {