Change the Rakefile to isolate the regular specs from the SWT specs.

Original implementation does this

````
- spec
-- spec:all
--- spec:shoes
----> This runs Shoes specs `spec/shoes/**/*_spec.rb`
--- spec:swt
---- spec:swt:all
-----> This runs SWT + Shoes specs `spec/{shoes,swt_shoes}/**/*_spec.rb`
````

There are two problems with this

1. Regular Shoes specs are run twice
2. The second time they are run in the same process as the SWT specs,
causing issues, because the SWT specs mess with global state, making
the other examples fail.

We really need to look into what the core issue is, but in the meantime
this is the sensible thing to do.

@bjoska & @arnebrasseur