Use proper PhantomJS library and run the tests as separate step

* add dependency for phantomjs

.drone.yml

@@ -1,5 +1,9 @@
 build:
-  backend:
+  jsunit:
+    image: morrisjobke/nextcloud-ci-jsunit:1.0.2
+    commands:
+      - ./autotest-js.sh
+  sqlite:
     image: morrisjobke/nextcloud-ci-php7:1.0
     commands:
       - git submodule update --init

build/package.json

@@ -16,7 +16,7 @@
     "karma-junit-reporter": "*",
     "karma-coverage": "*",
     "karma-phantomjs-launcher": "*",
-    "phantomjs": "*",
+    "phantomjs-prebuilt": "*",
     "jasmine-core": "~2.3.4"
   },
   "engine": "node >= 0.8"