Improved documentation for Scala 2.10.x dependencies

A build for 2.10.4 is not available - but it should not be needed.

Note: I didn't fully test this yet, but IIUC this is supposed to work. I opened this pull request for discussion, but if I got it right, this could be merged directly.
Depending on your preference, you might want to publish a build for 2.10.4, to explain the possible options differently, or to use 2.10.2 as a version suffix in the suggested SBT snippet (which should work just as well).