Changes to #2828 for Heroku work

[hone]

Changes to #2828

1. The middleman buildpack is redundant. Any ruby app can hook into an asset compilation step by just defining the task assets:precompile like Rails does. This just executes the build method for building the middleman app.
2. The static buildpack allows variable substitution using env vars using the syntax ${}. This change lets us set the env var EMBER_API_DOCS_URL to any instance we want to proxy to.

I deployed version of ember-api-docs to Heroku where I changed the rootURL to be /api-new/ to match the proxy. Fastboot works, but I ran into issues with pouchdb/couchdb when the JavaScript loads. I've been working with @sivakumar-kailasam to deploy a working version without a dependency on pouchdb: ember-learn/ember-api-docs#146.

Currently, you can see the progress here: https://ember-website-hone.herokuapp.com/api-new

As part of this work, I fixed a proxy redirect issue in the static buildpack wrt schemes, heroku/heroku-buildpack-static#61