update travis for SDK 28

.travis.yml

@@ -20,21 +20,21 @@ android:
     - tools # to get the new `repository-11.xml`
     - tools # see https://github.com/travis-ci/travis-ci/issues/6040#issuecomment-219367943)
     - platform-tools
-    - build-tools-27.0.3
+    - build-tools-28.0.3
     - android-22
-    - android-27
+    - android-28
     - extra-android-m2repository
     - extra-google-m2repository
     - extra-google-google_play_services
-    - addon-google_apis-google-27
+    - addon-google_apis-google-28
     - sys-img-armeabi-v7a-android-22
   licenses:
     - 'android-sdk-preview-license-52d11cd2'
     - 'android-sdk-license-.+'
     - 'google-gdk-license-.+'
 
 before_install:
-  - yes | sdkmanager "platforms;android-27"
+  - yes | sdkmanager "platforms;android-28"
   - mkdir -p "$ANDROID_HOME/licenses"
   - echo -e "\n8933bad161af4178b1185d1a37fbf41ea5269c55" > "$ANDROID_HOME/licenses/android-sdk-license"
   - echo -e "\n84831b9409646a918e30573bab4c9c91346d8abd" > "$ANDROID_HOME/licenses/android-sdk-preview-license"