Fix build by specifying SBT version
Fix build by specifying SBT version

The instructions in README.md did not work for me but worked for Cai. It
turned out I was using the latest SBT (0.13.7), while Cai was using
0.13.1. To avoid that ensure we get the same build environment, specify
the SBT version to use.

yfcai/CREG