Replace colorize gem with rainbow.

Colorize is a gem licensed under the GPLv2, so we can’t use it in GitLab without relicensing GitLab under the terms of the GPL. Rainbow is licensed under the MIT license and does the exact same thing as Colorize, so Rainbow was added in place of Colorize.

The syntax is slightly different for Rainbow vs. Colorize, and was updated in accordance.

The gem is still a dependency of Spinach, so it’s included in the development/test environments, but won’t be packaged with the actual product, and therefore doesn’t require we relicense the product.

An attempt at relicensing Colorize was made, but didn’t succeed as the library owner never responded.

Rainbow library: https://github.com/sickill/rainbow
Relevant issue regarding licensing in GitLab's gems: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3775