Usage of pify breaks reference to provider

[danfinlay]

At MetaMask we've been blocked from rolling out our new provider, because it breaks Drizzle, and we don't want to break every Drizzle app.

As documented in MetaMask/metamask-extension#5459:

• Drizzle pifies its provider, breaking the this reference.
  • Should make sure pify method receives correct context argument.
  • May also require patch in blocktracker-es5 2.3.2, which we don't own. (I'm not clear what this would be yet, will define later).

[OnlyOneJMJQ]

Thanks for bringing this to our attention @danfinlay. Closing with merger of #131.