Update travis configuration

Author: jdnichollsc

.travis.yml

@@ -66,14 +66,14 @@ android:
     - tools
     - platform-tools
     # The BuildTools version used by your project
-    - build-tools-26.0.2
+    - build-tools-28.0.3
     # The SDK version used to compile your project
-    - android-26
+    - android-28
     # Additional components
     - extra-android-m2repository
     # Specify at least one system image,
     # if you need to run emulator(s) during your tests
-    - sys-img-x86-android-26
+    - sys-img-x86-android-28
     - sys-img-armeabi-v7a-android-17
 
 before_install: