Add Node v8 and v10 to Travis config.

[Jimbly]

Update tests to work on Node v10.x.
Resolves #58
Thanks @paulrutter, code mostly from your PR. Other changes I just diffed with what's in nodejs/master and saw what they changed, and it seemed to resolve things.
All tests should pass on v6, v8, v10, and v11.

[paulrutter]

Thanks @Jimbly!
Shouldn't we also update the package.json version and publish it as new version to NPM?

[Jimbly]

I almost did... but then realized nothing has changed. The only changes we've made have been to Node.js and the test behavior, http-parser.js is unchanged.

[paulrutter]

Hehe, you're right indeed.
In the end it was quite some work, but we ended up changing nothing (in code) at all.