Enable cross-building for Scala 2.13

[zackthehuman]

Hi folks, I have a Play application that is on Scala 2.13 and I'm not able to use play-parseq because it only builds artifacts for 2.11 and 2.12. It appears that the 2.12 artifact is not compatible with 2.13 due to differences in how scala.Serializable is implemented (there are likely more issues, that's just one example).

I'm happy to send a PR if we're open to expanding the cross-building versions to include 2.13.

[FranklinYinanDing]

Hi Zack, I'm open to cross-build against Scala 2.13. However, Play 2.6 does not have Scala 2.13 artifacts, so in order to do this, a Play upgrade will need to happen at the same time. Although I imagine the work isn't too heavy.