Remove unnecessary dependencies

[PiotrWpl]

Fixes #3

[phillwiggins]

What's your thoughts on this?

So, my understanding and reasoning behind this was that to have this library available via DartPub, this was required?

[PiotrWpl]

When I installed yours plugin from DartPub it didn't worked, because of this issue #3
First I thought that was because of lack ios and android directories. But when You said that it's working for You when You copy plugin files into main project package, I did some tests, and realized that when I remove those dependencies in yours plugin .yaml file, then project is compiling with success. And it makes sense, because You are not using any native SDKs, and with those dependencies included in yaml file builder was trying to use them, and then error was appear.