fix(positioning): stop flicker when closed

When the dropdown is in an up position and it is closed we were
removing the positioning before hiding it which caused a flicker
while it moved down to it's natural bottom position. This change
sets the opacity to 0 before resetting the positioning style
attributes.

src/uiSelectDirective.js

@@ -383,6 +383,7 @@ uis.directive('uiSelect',
             }
 
             // Reset the position of the dropdown.
+            dropdown[0].style.opacity = 0;
             dropdown[0].style.position = '';
             dropdown[0].style.top = '';
             element.removeClass(directionUpClassName);