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.
Yea, I think using a relative require makes sense... Want to send such a pull request upstream? Then we can just sync it down.

(Feel free to leave this one open in case they point out a good reason for doing it this way...)