bump Scala versions (to 2.13.4 and 3.0.0-M2)

Author: SethTisue

.travis.yml

@@ -5,8 +5,8 @@ import: scala/scala-dev:travis/default.yml
 language: scala
 
 scala:
- - 2.13.3
- - 3.0.0-M1
+ - 2.13.4
+ - 3.0.0-M2
 
 env:
  - ADOPTOPENJDK=8