Add support for Node.js 6

[ppitonak]

When I run tests npm test, I get this error:

$ npm test

> mock-fs@3.8.0 pretest /home/ppitonak/workspace/mock-fs
> eslint benchmarks lib test

> mock-fs@3.8.0 test /home/ppitonak/workspace/mock-fs
> mocha --recursive test

/home/ppitonak/workspace/mock-fs/lib/index.js:36
  throw new Error('Unsupported Node version: ' + nodeVersion);
  ^

Error: Unsupported Node version: 6.0.0
    at Object.<anonymous> (/home/ppitonak/workspace/mock-fs/lib/index.js:36:9)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Module.require (module.js:466:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/home/ppitonak/workspace/mock-fs/test/integration/filecount.spec.js:4:12)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Module.require (module.js:466:17)
    at require (internal/module.js:20:19)
    at /home/ppitonak/workspace/mock-fs/node_modules/mocha/lib/mocha.js:219:27
    at Array.forEach (native)
    at Mocha.loadFiles (/home/ppitonak/workspace/mock-fs/node_modules/mocha/lib/mocha.js:216:14)
    at Mocha.run (/home/ppitonak/workspace/mock-fs/node_modules/mocha/lib/mocha.js:468:10)
    at Object.<anonymous> (/home/ppitonak/workspace/mock-fs/node_modules/mocha/bin/_mocha:403:18)
    at Module._compile (module.js:541:32)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:456:32)
    at tryModuleLoad (module.js:415:12)
    at Function.Module._load (module.js:407:3)
    at Function.Module.runMain (module.js:575:10)
    at startup (node.js:159:18)
    at node.js:444:3
npm ERR! Test failed.  See above for more details.

[tschaub]

Node 6 support in mock-fs@3.9.0 (thanks @tmcw).