Let bin scripts use the correct npm module for esprima.

The scripts included with esprima in `bin` were importing the upstream esprima module, which goes by a different name in npm. This caused anybody using esprima from the command-line to confusingly lack JSX support.
Seems fine. Though actually I think upstream should make this require relative (`require('../esprima')`) and then we wouldn't need to be different. I don't know what the Right Thing™ is though.