Enable reactphp/async to work on PHP 5.3.6; ensure errors in correct order

Enable the `parallel` function to work on PHP 5.3.6 (running the unit tests just emitted segfaults).

Also I noticed that the order of `$errors` would not necessarily match up with the order of parallel tasks so I fixed that too.

(I tried to use Travis to recreate the bug on 5.3.6, but it doesn't support that version :confused:).