Add scala 3.0.0

dropping 3.0.0-RC2 and 3.0.0-RC3

Author: Philippus

.travis.yml

@@ -5,8 +5,7 @@ import: scala/scala-dev:travis/default.yml
 language: scala
 
 scala:
-  - 3.0.0-RC3
-  - 3.0.0-RC2
+  - 3.0.0
   - 2.11.12
   - 2.12.13
   - 2.13.5