Call replace on string instead of {string.js} object

[spence]

To reproduce:

test.js

// Invoke section of babel-polyfill code that resets Object.prototype
import 'babel-polyfill';
for (let _ of []) { _; }

import S from 'string';
S('Test').slugify().s;

$ nvm use v0.10.24
$ npm install babel-cli babel-preset-es2015 babel-polyfill string
$ node_modules/.bin/babel --presets es2015 test.js > test.es5.js
$ node test.es5.js

/git/test/node_modules/string/lib/string.js:134
      var s = this.replace(/[^A-Za-z0-9\[\] ]/g, function(x) { return latin_ma
                   ^
TypeError: Object Test has no method 'replace'
    at S.latinise (/git/test/node_modules/string/lib/string.js:134:20)
    at S.slugify (/git/test/node_modules/string/lib/string.js:413:37)
    at Object.<anonymous> (/git/test/test.es5.js:20:31)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:902:3

[jprichardson]

Please add a test and squash the package.json change.