Remove esprima-six in favour for esprima

The former still has a lot of issues parsing, and will break on bundling modules such as `escodegen`.

Downgrading to esprima fixes this issue, at the expense of ES6 support. The same has been done for the following modules:

* astw (http://git.io/3qnK6A)
* detective (http://git.io/GeWevg)
* syntax-error (http://git.io/j7gJZg)

This should also probably be a major version bump to 1.0.0 too.

Thanks! :)