Cross-compile to 2.13.0 as well
Cross-compile to 2.13.0 as well

When including `gremlin-scala` as a dependency I want to have an option
to include a version compiled to a specific Scala version.
Currently, including `gremlin-scala` means that `scala-reflect` 2.13.1
joins the party always which is not necessairly desired, due to binary
incompatibilities between 2.13.0 and 2.13.1.

mpollmeier/gremlin-scala