Locked npm version on travis

npm/cli#611

Author: aalavandhan

.travis.yml

@@ -8,6 +8,7 @@ cache:
     - node_modules
     - $(npm config get prefix)/bin/ganache-cli
 before_install:
+  - npm install -g npm@6.4.1 # Locking npm to 6.4.1
   - npm install -g npx
 script:
   - ./scripts/setup-ci.sh