support non-es6 node consumers

[cesine]

We found that our installs were failing because a patch version introduced let mde@228d8e4#r40755379

Before:

46.80s$ npm install 
npm WARN package.json [email protected] No license field.
npm WARN deprecated [email protected]: you are strongly encouraged to use other testing options
npm WARN deprecated [email protected]: CoffeeScript on NPM has moved to "coffeescript" (no hyphen)
npm WARN deprecated [email protected]: express 2.x series is deprecated
npm WARN deprecated [email protected]: connect 1.x series is deprecated
npm WARN deprecated [email protected]: Legacy versions of mkdirp are no longer supported. Please update to mkdirp 1.x. (Note that the API surface has changed to use Promises in 1.x.)
npm WARN engine [email protected]: wanted: {"node":">=4"} (current: {"node":"0.12.18","npm":"2.15.11"})
> [email protected] postinstall /home/travis/build/cesine/eco/node_modules/nodeunit/node_modules/ejs
> node ./postinstall.js
/home/travis/build/cesine/eco/node_modules/nodeunit/node_modules/ejs/postinstall.js:9
let envDisable = isTrue(process.env.DISABLE_OPENCOLLECTIVE) || isTrue(process.
^^^
SyntaxError: Unexpected strict mode reserved word

This creates a branch to support 2.x without impacting master (3.x) since master has a major version bump which dropped support for older node versions 2b56f6e