Disable flutter beta test

(cherry picked from commit 17fed89)
(cherry picked from commit 593c446)
(cherry picked from commit cff2822)
(cherry picked from commit e156fba)
(cherry picked from commit e555552)

Author: abnegate

.travis.yml

@@ -27,7 +27,7 @@ env:
   - SDK=FlutterStable
   - SDK=Go112
   - SDK=Go118
-  - SDK=FlutterBeta
+#  - SDK=FlutterBeta
   - SDK=KotlinJava8
   - SDK=KotlinJava11
   - SDK=KotlinJava17