Update .travis.yml

Author: kbiakov

.travis.yml

@@ -13,6 +13,7 @@ android:
     - build-tools-25.0.3
     - android-25
     - extra-android-m2repository # Design Support library
+    - android-21
     - sys-img-armeabi-v7a-android-21 # system image (emulator)
   licenses:
   - android-sdk-license-.+