Fix drone yaml file

.drone.yml

@@ -1,14 +1,12 @@
 pipeline:
   build:
-   image: node:${NODE_VERSION}
+    image: node:${NODE_VERSION}
     commands:
       - npm install
       - npm run test
 
 matrix:
- NODE_VERSION:
-   - latest
-   - "8"
-   - "7"
-   - "6"
-   - "4"
+  NODE_VERSION:
+    - latest
+    - "9"
+    - "8"