Add bundler

When I first cloned down your project I wasn't able to run the app because I didn't have the colorize gem installed. You can use [bundler](http://bundler.io/) to specify all of the dependencies for your project.

I recommend checking out the website for a more detailed description but basically you create a `Gemfile` in the root of your project, tell it to use https://rubygems.org as the source for installing the dependencies, then you list each of the gems that your project depends on. (as shown in this pull request under the ["Files changed" tab](https://github.com/SpencerCDixon/theberrygame/pull/1/files)).

It's reasonable to expect that most people will have the bundler gem already installed.

This way, people can pull down your project and run `bundle install` to install all of the gems that your project depends on.

You can merge this pull request into your project or set it up yourself for more practice.