update ci config

Author: sw926

.travis.yml

@@ -11,10 +11,12 @@ android:
     - build-tools-25.0.3
     - build-tools-26.0.1
     - build-tools-26.0.2
+    - build-tools-28.0.3
 
     # The SDK version used to compile your project
     - android-25
     - android-26
+    - android-28
 
     # Additional components
     - extra-android-m2repository