specify branch that should be build

.drone.yml

@@ -22,3 +22,6 @@ pipeline:
       - ANDROID_TARGET=android-23
       - ANDROID_ABI=armeabi-v7a
       - LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/:/opt/android-sdk-linux/tools/lib64/gles_mesa/
+
+branches:
+  - master