BabelJS : Why transpilation is not working?

Question

I want to use babelJs to transpile javascript source from es6 ( or es7 ) to es5. So i created a project. Here my package.json file :

{
  "name": "myapp",
  "version": "1.0.0",
  "description": "my front end app",
  "main": "gulpfile.js",
  "directories": {
    "test": "test"
  },
  "dependencies": {},
  "devDependencies": {
    "babel-cli": "^6.10.1"
  },
  "scripts": {
    "build-js": "babel app/src/js -d app/libs/",
  },
  "author": "",
  "license": "ISC"
}

I create a javascript file :

var sayHello = name => {
    return "Hello: " + name;
};
var result = sayHello("Sandeep"),
    message = document.createElement("H1");
message.innerText = result;
document.body.appendChild(message);

When i run my task build-js , the output file contains the same content that input file. The transpilation did not happen. Is there anything to do else ? Thanks