Please add ES5 compatibility

[realtaraharris]

In order to use this module in an ES5 environment (Safari 9), I had to add a little snippet to node_modules/isemail/package.json:

  "browserify": {
    "transform": [["babelify", { "presets": ["es2015"] }]]
  }

Please consider adding this to your package.json. (Also, thanks for maintaining this module!)

[arb]

It is up to the developer to make the code work in their environment. Across the org, we do not include a single line of cleint-side transformation code.

Please see...
hapijs/joi#528
hapijs/boom#58

And many other issues in the organization related to this.