Add sbt-dependency-graph plugin

Don't have many dependencies in this project, but when trying out 2.10.1-RC1
SBT started warning about incompatibilities and this was the easiest way
to find out what was bringing in a 2.10.0-specific jar.
Use the canonical Block constructor in AutomaticJsonCodecBuilder

The helper Block(Tree*) constructor is deprecated starting in 2.10.1.