Various quality of life fixes

- Pinned the versions of the plugins used in each phase to their latest.
- The initial phase of the build now invokes the 'add-source' goal of scala-maven-plugin. This
  allows other plugins that use the project source dirs (for example, maven-source-plugin) to
  properly work out of the box without having to explicitly add a <sourceDirectory> element to the pom.
- Made scalatest and junit provided dependencies for surefire-scala