update version Android playground version to 21. The latest binary apk version is 21.
diff --git a/android/playground/build.gradle b/android/playground/build.gradle
index 5f16c34..b66dd9b 100755
--- a/android/playground/build.gradle
+++ b/android/playground/build.gradle
@@ -13,8 +13,8 @@
         applicationId "org.apache.weex"
         minSdkVersion project.minSdkVersion
         targetSdkVersion project.targetSdkVersion
-        versionCode 20
-        versionName "0.20.0"
+        versionCode 21
+        versionName "0.21.0"
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
         multiDexEnabled true
         ndk{