Update Shapeoko 2 defaults

The docs show a different set of defaults for the Shapeoko 2. this PR updates the following in `defaults.h` to bring them in line with the [official documentation](http://docs.shapeoko.com/software.html):

* feed rate (800 -> 500)
* seek rate ( 250 -> 500)
* acceleration (15 -> 25)
* step pulse ms (10 -> 30)