Rake Clean

[jasonkarns]

Bundler's built-in rake tasks add pkg/* to rake's CLOBBER list
(so rake clobber removes them).
That's great for .gem that are created via rake build because bundler's rake task writes the .gem to pkg/.
However, by default if you run gem build instead of rake build, you'll end up with the .gem written in the root (next to where the .gemspec is).
This adds *.gem to rake's CLOBBER list so that rake clobber will properly clean any built gems regardless if they were built via rake or gem.

(It also moves the default task to the top of the file rather than the bottom.
That's the conventional location inherited from make whose default task is automatically the first listed target.)