Updates needed to work with Stack versions >= 2.1.1

[malob]

It looks like Stack changed a bunch of stuff in version 2 that effects this project. So far they all seem to relate to the config.yaml file:

• package-indices now requires hackage-security (see here). I think a workaround for this could be:

  package-indices:
  - name: Hackage
     download-prefix: $MIRROR_URL/packages/
     http: $MIRROR_URL/01-index.tar.gz
     hackage-security:
       keyids: [""]
       key-threshold: 0        
  

• lts-build-plans and nightly-build-plans are no longer supported. As far as I can tell, there's no alternative/workaround for this. It looks like one can workaround this on a per project basic though by specifying a snapshot location in stack.yaml (see here). So adding a note about that to the readme might be helpful.