Change default RTD config to use pip and extras-require

[astrofrog]

We should update the package template to match the core package in terms of RTD configuration - basically having a configuration that uses pip and relies on the extras-requires (which can default to sphinx-astropy)

build:
    image: latest

python:
  version: 3.6
  pip_install: true
  extra_requirements: ['docs']

[pllim]

Fixed in #438