Correct Make definitions so tutorial building works as advertised

Provide a CHISEL_DEFAULT_VERSION ("latest.release") for non-developer
builds so users grabbing the repository and following instructions
aren't confused by:
[info]  [SUCCESSFUL ] com.github.scct#scct_2.10;0.2!scct_2.10.jar (8159ms)
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  ::          UNRESOLVED DEPENDENCIES         ::
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::
[warn]  :: edu.berkeley.cs#chisel_2.10;2.3-SNAPSHOT: not found
[warn]  ::::::::::::::::::::::::::::::::::::::::::::::