Use babel-preset-env in place of babel-preset-es2015

[prescottprue]

If you're reporting a bug, please include input code, output documentation,
a description of what you expected to happen, and what happened instead.

When installing through npm there is an npm warning that babel-preset-env should be used instead of babel-preset-es2015:

Switching would silence warning.

• What version of documentation.js are you using?:
  Happens with both v4.0.0-beta.15 and v5.3.2 both

• How are you running documentation.js (on the CLI, Node.js API, Grunt, other?):
  CLI