Switch to Facebook's esprima fork

Facebook's esprima fork is maintained at a higher frequency and already
has computed keys implemented which eliminates the need for a custom
fork.

This also makes it easier to integrate with tool chains such as esnext
(which may switch to esprima-fb very soon for similar reasons).
Merge pull request #2 from steckel/steckel/esprima-fb

Switch to Facebook's esprima fork