Don't run prepublish on npm install --production.

* `npm install` usually installs devDependencies+regular deps
* `npm install --production` only installs non-devDependencies
* `npm install` always runs `prepublish` scripts
* prepublish scripts often rely on dev dependencies
* *Problem*: `npm install --production` will try invoke missing devDependencies

This pull request prevents prepublish scripts from running when the `--production` flag is present.

Seems to make sense to me. 

Although…
People tend to flip out when things change though so it may not be worth rocking the boat. 

I wonder if "breaking" behaviour changes such as this should obey semver conventions, you could consider npm's command line it's api i.e. when will npm hit 2.x?