Update to android 24

Author: jdnichollsc

.travis.yml

@@ -45,7 +45,7 @@ matrix:
       sudo: required
       jdk: oraclejdk8
       before_script:
-        - echo no | android create avd --force -n test -t android-21 -b armeabi-v7a
+        - echo no | android create avd --force -n test -t android-24 -b armeabi-v7a
         - emulator -avd test -no-audio -no-window &
         - android-wait-for-emulator
       before_install:
@@ -65,7 +65,7 @@ android:
     - build-tools-28.0.3
     - android-28
     - extra-android-m2repository
-    - sys-img-armeabi-v7a-android-21
+    - sys-img-armeabi-v7a-android-24
 
 before_install:
     - sudo pip install --upgrade pip