[3.x] Fix Story & Task option merge priority

[Tofandel]

See #248

Consider the following

@servers(['local' => 'localhost', 'dev' => 'someserver'])

@story('deploy', ['on' => 'dev'])
task_1
task_2
@endstory

@task('task_1')
echo 'task_1';
@endtask

@task('task_2', ['on' => 'local'])
echo 'task_2';
@endtask

Currently task_2 will be executed on the remote server instead of the expected localhost because the story options have priority over the task options but the behavior should be the opposite because a task is more specific than a story

After this PR the task options will correctly override the story options and task_1 will be executed on dev and task_2 on local

[taylorotwell]

I don't plan to make any breaking changes to this library since there is an easy work around.

[Tofandel]

@taylorotwell this fix is required though because first it is not that easy to do when you have 20 tasks on a file and second the workaround is not applicable when using file inheritance between projects (which I'm doing) so it's becoming a big problem (eg story in one file extending tasks in another file)

[Tofandel]

There is the possibility to add an option to story that would change the inheritance priority that would avoid a BC break

[driesvints]

We already tried this. See #157 and then had to revert it because of #186. We probably will never change this sorry.