shoes.gemspec at top of project

Fixes #956 by moving `shoes.gemspec` to the top of the project.

Has a little oddity in how the root project `rake build` now builds `shoes.gem`, with the others as sub-projects, but the `*:all` tasks do what you'd expect.

Fiddles with where we pull in the version to draw from the `./VERSION` file for `shoes`. This introduces a `rake update_versions` task to write that version to the `version.rb` files in each subproject (since those still need to be defined in a Ruby file to have a runtime constant). This is patterned after how Rails structures itself--figure they know how to set up a meta-gem if anyone else.

Would appreciate any Linux testing, and will see if I can cover Windows before we land this.