Replace brunch with webpack

[StephaneRob]

Phoenix will use webpack in the next release phoenixframework/phoenix#2779

[brienw]

@rjanja @StephaneRob @willfore Should we be concerned about only supporting webpack now, or would we be better off supporting both and having some sort of auto-detection for a while?

Maybe the real answer is to just have a line in the README that says "Use bootleg_phoenix version 0.2.0 for phoenix versions older than 1.4"

thoughts?

[willfore]

I would say updating the README would be a good solution. Since webpack will be the standard in 1.4 going forward.

[rjanja]

I’m also in favor of the documentation change. As this package exists to provide the most common solution for users deploying a Phoenix app, it makes sense to keep things simple and just support the latest. For other use cases a custom task can always be hand-rolled.