Fix Travis script

Author: vladistan

.travis.yml

@@ -28,8 +28,8 @@ install:
   # (gulp e2e commands below invoke the corresponding gradle tasks
   # to congifure the java middle tier)
 
-  - npm cache clean
-  - npm install -g npm@2.1.10
+  - npm cache verify
+  - npm install -g npm@6.9.0
   - npm install -g gulp
   - npm install