Generate versionCode from timestamp.

.drone.yml

@@ -18,6 +18,13 @@ steps:
   - echo "keyAlias=key0" >> keystore.properties
   - echo "storeFile=../AndroidKey" >> keystore.properties
 
+- name: generate versionCode
+  image: busybox
+  commands:
+  - touch version.properties
+  - let timestamp=$(date +%s)/10 
+  - echo "versionCode=$timestamp" >> version.properties
+
 - name: build
   image: mingc/android-build-box
   commands: