Issue Description

Reaction prerender feature does not work with prerender.io

Steps to Reproduce

1. Get a token from Prerender by signing up/logging in to their service.
2. Using this token, set your environment variables as:

PRERENDER_TOKEN=token
PRERENDER_SERVICE_URL=http://service.prerender.io/

3. In your prerender.io dashboard, the message "We haven't seen a request with your Prerender token yet." is not removed despite correct set up in Step 2.

Possible Solution

Update server/startup/prerender.js to require PRERENDER_SERVICE_URL instead of PRERENDER_HOST

See https://github.com/prerender/prerender-node

Versions

Node: 8.11.2
NPM: 5.6.0
Meteor Node: 8.11.2
Meteor NPM: 5.10.0
Reaction CLI: 0.29.0
Reaction: 1.12.1
Reaction branch: master