Fix dependency for machines behind proxies

Hi there

Since the update of the `metalsmith` package my dependency tree looks like that:

* `metalsmith@1.0.1`
  * `gnode@0.1.0` 
    * `regenerator@0.6.10` 
      * `defs@1.0.1`
        * `esprima@git://github.com/ariya/esprima.git#harmony`

Since I'm using npm behind a corporate proxy which doesn't allow `git://` requests, the installation of the packages fails because of the dependency to esprima.

I suggest we change `git://` to `git+https://`, so the package can be fetched via `https`. Then we could move the `1.0.1` tag to the new commit. That way no changes to the dependant packages are necessary.