Use pre-publish rather than pre-install

Using a prepublish script rather than a preinstall script makes this package a much lighter dependency by not requiring cherio and superagent to be installed at runtime.  It also makes it much safer to have as a dependency because you know that if you get the same version of the package, it will have the same list of elements.  If you depend on the github repo directly it should still run the prepublish script so it will still update, and when you need to update it you can still just update the version number then run `npm publish`.

P.S. if you ever do need to do something on install you can use the postinstall hook and then you wouldn't need to manually add `npm install cherio superagent`