Removed Dependency On Heroku's Docker Image

[tompurl]

This is related to issues #34 and #35.

I and others were not able to build the dev environment using the Heroku Docker image. When I looked into it I found that we would have to either use a very old version of the parser library or hack the Heroku Docker image, both of which sounded unmaintainable.

I therefore did the following:

• Made code and doc changes that make it possible to build a dev environment with "plain-old" Docker.
• Made doc changes that tell the user that the code base can be developed using a non-Docker-based rails setup.
• Made doc changes that show developers how to push code to a "prod" environment without using Heroku's Docker image.

[samuelcole]

lgtm!

excellent documentation!