Un-comment remaining Gemfile dependencies

In the spirit of this project’s rules:

Once the rails new command completes, the project is forbidden from adding or removing a Rubygems, NPM, or Yarn dependency.

This commit installs the exceptions to that rule:

The only exceptions to this rule are dependencies in our Gemfile that were introduced by a rails new command. For example, the bcrypt gem, the image_processing gem, and the redis gem can be made available as dependencies, since they’re generated by rails new as commented-out lines in the Gemfile.

To get those dependencies out of the way, this commit pre-emptively installs them, closing the door on future dependencies.