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).
I wouldn't say it doesn't work, they just wanted it to be visible somewhere in the .sass files. Having it output the comment to the compiled css would be even better though. We should do this in Bitters too then.