fix($animate): insert elements at the start of the parent container instead of at the end

With 1.2.x, $animate.enter and $animate.move both insert the element at the end of the provided
parent container element when only the parent element is provided. If an after element is provided
then they will place the inserted element after that one. This works fine, but there is no way to
place an item at the top of the provided parent container using both these functions.

Closes #4934
Closes #6275

BREAKING CHANGE: $animate will no longer default the after parameter to the last element of the parent
container. Instead, when after is not specified, the new element will be inserted as the first child of
the parent container.

To update existing code change any instances of $animate.enter() or $animate.move() from:

$animate.enter(element, parent);

to:

$animate.enter(element, parent, angular.element(parent[0].lastChild));