Updated module for new React version 15.5

[emoriarty]

The current lib. is throwing the error shown below because an old version of react-element-to-jsx-string is being used. Exactly the version is 2.0.4. The current version is 6.4.0.

Error: Cannot find module 'react-addons-test-utils'
    at Function.Module._resolveFilename (module.js:470:15)
    at Function.Module._load (module.js:418:25)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/enric/Workspace/learning/tape/node_modules/react-element-to-jsx-string/index-dist.js:18:29)
    at Module._compile (module.js:571:32)
    at Module._extensions..js (module.js:580:10)
    at Object.require.extensions.(anonymous function) [as .js] (/Users/enric/Workspace/learning/tape/node_modules/babel-register/lib/node.js:152:7)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
not ok 1 no plan found
not ok 2 no assertions found

Also, I've modified the react-addons-test-utils because has been deprecated from React v15.5.0. Now those utils are in react-dom module. The test is passing properly.