Fix JSON in README.md

[okcoker]

If you're doing any sort of JSON.parse of your babelrc file, these keys need quotes. Otherwise you run into something like:

undefined:24
            globals: ["Error"],
            ^

SyntaxError: Unexpected token g in JSON at position 591
    at JSON.parse (<anonymous>)
    at Object.<anonymous> (/path/file.js:38:26)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Function.wrappedLoad [as _load] (/path/node_modules/newrelic/lib/shimmer.js:344:38)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)