utils: do not use internal Node API

process.binding() is undocumented internal API, and not intended for use
in userland modules.

In this case, it's also unnecessary, since vm.Script is exactly equal to
the old binding('evals').NodeScript.  However, the
process.binding('evals') is gone as of Node 0.11.
Merge pull request #229 from isaacs/master

utils: do not use internal Node API