chore(slugbuilder): bump heroku buildpacks
docs(using_buildpacks): add sidenote on Scala buildpack

Play v2.0 apps and above use the Scala buildpack, which requires
a minimum of 512MB for to execute sbt, which invokes the JRE.
Merge pull request #2769 from bacongobbler/bump-buildpacks

chore(slugbuilder): bump heroku buildpacks