Add version to main manifest file - Closes #430

For those who don’t use Bourbon in Rails or with some sort of package manager and instead simply download the Bourbon files, it can be difficult to tell what version they are using. This adds the typical convention of included the version in the main manifest.

This is exactly how [Bitters does it](https://github.com/thoughtbot/bitters/blob/master/app/assets/stylesheets/_base.scss).
The real benefit is providing insight into which version of Bourbon your app is using regardless of rails/non-rails config. Regular CSS comments will provide much better insight down the compilation funnel.