Prefer vm instead of process binding evals

`require('vm')` should be preferred over `process.bindings('evals')`. The
latter has been removed in node 0.11.
Not since the rewrite for node 0.11, afaik. http://nodejs.org/docs/v0.11.13/api/vm.html#vm_class_script. The constructor version works on node 0.10 too, but can't tell for sure if there are any drawbacks with it.