fix travis for node 0.12: lock ejs to before let was introduced

[cesine]

Steps to reproduce

Before https://travis-ci.org/github/caolan/nodeunit/builds/710077144

https://travis-ci.org/github/caolan/nodeunit/jobs/710077156

$ npm install

ejs@2.7.4 postinstall /home/travis/build/caolan/nodeunit/node_modules/ejs
node ./postinstall.js
/home/travis/build/caolan/nodeunit/node_modules/ejs/postinstall.js:9
let envDisable = isTrue(process.env.DISABLE_OPENCOLLECTIVE) || isTrue(process.
^^^
SyntaxError: Unexpected strict mode reserved word
at exports.runInThisContext (vm.js:73:16)
at Module._compile (module.js:443:25)
at Object.Module._extensions..js (module.js:478:10)
at Module.load (module.js:355:32)
at Function.Module._load (module.js:310:12)
at Function.Module.runMain (module.js:501:10)
at startup (node.js:129:16)
at node.js:814:3

After https://travis-ci.org/github/caolan/nodeunit/builds/710097661

Root cause

mde/ejs@v2.7.2...v2.7.3 introduced a let before ejs@3.x dropped support for older node versions

[mreinstein]

@caolan it might be good to officially archive this repo to prevent more PRs and things coming in. People don't seem to be heeding the note at the top of the README regarding deprecated state of this project.

[cesine]

I saw the note, I made the PR because I think it fits this: https://www.npmjs.com/package/nodeunit

We've kept it working on new versions of node, and sometimes merged small PRs that help teams relying on nodeunit.

Officially archiving it would result in fragmentation (I can show you more in a hangout)

I'm not able to update one of our repos to use another test runner, so helping make tiny fully tested PRs is easier for me.

[mreinstein]

The whole point of deprecating this project is to inform you that this library is extremely out of date and won't be further supported. It is essentially "locked" as is so that other ancient software that relies on this continues to function.

There are literally dozens of modern and supported unit testing frameworks out there. Please use one of them.

[caolan]

@mreinstein happy to archive if you think that's appropriate.