Add drone CI

.drone.yml

@@ -0,0 +1,13 @@
+kind: pipeline
+name: default
+
+steps:
+    -   name: build
+        image: cirrusci/flutter:stable
+        commands:
+            - flutter doctor
+            - flutter pub get
+            - flutter pub upgrade
+            - flutter packages pub run build_runner build --delete-conflicting-outputs
+            - flutter analyze --no-pub --no-current-package lib/
+            - flutter build apk