Restore continuations on 2.10.x, remove on 2.11.x.

In the past, the continuations library was built alongside
the scalalib. This commit restores this for 2.10.

However, it also removes definitively the continuations
library from builds for 2.11.x, as it is out-sourced in a
separate repo anyway in Scala itself.
Build for 2.10.4.

And remove build from 2.10.4-RC3.
Merge pull request #387 from sjrd/build-2.10.4-and-2.11.0-RC3

Build for 2.10.4