Babel deprecated

Running the `build` command, I get:

```
You have mistakenly installed the `babel` package, which is a no-op in Babel 6.
Babel's CLI commands have been moved from the `babel` package to the `babel-cli` package.

    npm uninstall babel
    npm install --save-dev babel-cli
```

Installing `babel-cli` and `babel-preset-env` solved the issue.