Revert "Change default scala version to 2.10.4."

This reverts commit 08ae7e190d3ff59a2f3ab0dc396305050642b697.

We decided that it was worth the pain of "accidentally" running into 2.9
errors when coding with 2.10 in mind, especially considering the
forthcoming work that will happen in 2.10 land on Lift 3. See
https://github.com/lift/framework/pull/1536 for more.