Update sbt and jgit version

This bumps sbt to use the latest 0.13.2, as well as bumps jgit to use the newer 3.3.0 version. Also bumped the version to 0.6.4-SNAPSHOT so it's ready to deploy. Context for this PR is I am using jgit directly in a project that needs APIs from v3, but I also want to use sbt-git APIs as well, which currently uses v2 of jgit. Would like to get this new version deployed if possible so I can rely on sbt-git directly.