chore($q): rename `promise.always` to `promise.finally`

BREAKING CHANGE: the `always` method no longer exists, but has been replaced by the
functionally equivalent `finally`. The reason for this change is to align `$q` with
the Q promises library, despite the fact that this makes it a bit more difficult to
use with non-ES5 browsers, like IE8.

To migrate the code follow the example below:

Before:

$http.get('/foo').always(doSomething);

After:

$http.get('/foo')['finally'](doSomething);