add Scala 3.0.0-RC3 (and drop RC1) (#372)

Author: SethTisue

.travis.yml

@@ -5,8 +5,8 @@ import: scala/scala-dev:travis/default.yml
 language: scala
 
 scala:
+  - 3.0.0-RC3
   - 3.0.0-RC2
-  - 3.0.0-RC1
   - 2.11.12
   - 2.12.13
   - 2.13.5