update npm version to support npm ci

Author: torresxb1

.appveyor.yml

@@ -93,7 +93,7 @@ for:
       - sh: "source ${HOME}/venv${PYTHON_VERSION}/bin/activate"
       - sh: "rvm use 2.5"
       - sh: "nvm install ${nodejs_version}"
-      - sh: "npm install npm@5.6.0 -g"
+      - sh: "npm install npm@5.7.0 -g"
       - sh: "npm -v"
       - sh: "echo $PATH"
       - sh: "java --version"