Fix overflow pre tag index.md

[lopezac]

In the docs, at h2#testing-on-localhost second pre child, the code snippet literally breaks and goes outside pre, div and everything. The same happens for h2#promise-and-generator-support third pre child, the code snippet breaks and goes outside the pre and everything. https://visionmedia.github.io/superagent/#testing-on-localhost https://visionmedia.github.io/superagent/#promise-and-generator-support

[titanism]

v8.0.4 released https://github.com/visionmedia/superagent/releases/tag/v8.0.4, thank you