Remove therubyracer as a dependency

[butzopower]

Currently therubyracer dependency requires quite a lot of configuring to get setup on OS X Catalina, as well as requiring a very out of date version of V8 to be installed.

Looking through the commits that included it as a dependency, it looks though it was originally brought in to support gocd, which I'm uncertain is actually a requirement anymore.

Looking at the actual built apps bookbinder creates, I'm not certain why it is a dependency of the created rack app, either, as I'm not even sure how it would be invoked since sprockets or execjs aren't included, nor are there any direct references to it.

For these reasons, I've removed the dependency outright. Potentially it could be replaced with mini_racer, which seems to build more easily on OS X, but again, I'm not sure any ruby JS runtime is required.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[rhoughton-pivot]

Hey, can we move forward on this? I'm unable to run bookbinder on my mac at this time.