fix: use legacy syntax for decorators

[anehx]

This commit rewrites the decorators withParachute and queryParam to
use the stage 1 legacy decorator syntax since ember uses that syntax for
now. For more infos see https://github.com/ember-decorators/ember-decorators/blob/master/README.md#specs.

This will fix the usage of ember-parachutes decorators with newer versions of ember / ember-decorators.

[anehx]

This also drops support for node 6 since it's EOL is 2019-04-30 and ember-decorators v6+ doesn't support it

[anehx]

@offirgolan I don't really see the reason for the failing tests.. Maybe cleaning the Travis CI cache would help?

[anehx]

Sadly I just learned that this doesn't really work the way I intended it.. The routes setup hook which initializes the QPs values uses a computed property _qp which uses the owner.lookup method to get the controller definition. If the controllers queryParams property is defined via Controller.reopen it does not exist then (not quite sure why, since controllers are supposed to be a singleton).

Long story short, this won't work properly. I don't have any more ideas how to solve it. If someone has an idea I'm very willing to try it out..