Add missing require and package dependency for color-theme

This commit allows color-theme to be automatically installed when color-theme-heroku is installed.

It also autoloads the 'color-theme-heroku function so that users do not need to explicitly require 'color-theme-heroku first.

BTW, you might consider making an Emacs 24 version of this theme, thus removing the dependency on color-theme. :-) For an example of a theme which has versions for both `color-theme` and the Emacs 24 built-in theme system, see https://github.com/juba/color-theme-tangotango/