Replace circular-json with flatted

[cktang88]

Closes #35 .

[polarathene]

So is the CI error due to Mocha switching to ES6+ syntax but flat-cache enforcing "use strict" in the tests? How can that be corrected? or can it be safely ignored for the PR to get merged?

/home/travis/build/royriojas/flat-cache/node_modules/mocha/bin/mocha:10
const spawn = require('child_process').spawn;
^^^^^
SyntaxError: Use of const in strict mode.
    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
npm ERR! Test failed.  See above for more details.
The command "npm test" exited with 1.

[coreyfarrell]

The error is due to current version of mocha requiring node.js >= 4.0.0. IMO .travis.yml should be updated to remove v0.10, probably add v8 and v10.