Add "mime-types" to dependencies.

[chellman]

I had to install the mime-types gem before the nanoc autocompile would work, so that should probably be added to the dependency list.

[rmurphey]

If the eventual plan is to deploy this using nanoc or another ruby tool, then dependencies ought to be specified using bundler http://gembundler.com/

[addyosmani]

I agree with @rmurphey on this one. As we've using nanoc at the moment it makes the most sense.

[ajpiano]

Long since resolved :)