Change geckodriver version to be able to install it behind proxy

Following PR #1954

@natabene

Ecommerce has a dependency with `geckodriver`. This library uses `got` dependency to download its `.tar.gz` file. The `got` dependency does not work behind a proxy. So the installation of ecommerce fails with:

```
RequestError: connect ECONNREFUSED 192.30.253.113:443
at ClientRequest.<anonymous> (/edx/app/ecommerce/ecommerce/node_modules/got/index.js:69:21)
```

The `got` developers does not have in their roadmap to make it work behind proxy: https://github.com/sindresorhus/got/issues/560

> Proxy support is really hard to get right and not something I'm interested in spending time on and I don't want to bloat Got with dependencies to support it.

The `geckodriver` version used in `open-release/hawthorn.master` is `1.6.1`, where the download is made by one of these URLs:
https://github.com/vladikoff/node-geckodriver/blob/v1.6.1/index.js#L14-L18

In the version `1.7.1` of `geckodriver`, the URL is configurable:
https://github.com/vladikoff/node-geckodriver/blob/v1.7.1/index.js#L14-L23

The workaround to fix ecommerce installation behind proxy is to upgrade `geckodriver` to `1.7.1`. The process of installation should be downloading the `geckodriver` first to the same network as the proxy, place the file in a URL the server can access and define `GECKODRIVER_CDNURL` in the environment before installing it.

I've made this PR to `master`, but I would like this to be cherry-picked to `open-release/hawthorn.master` if possible, because our installation is made behind proxy. Without this fix, we can not install Open edX at all. Is it possible to contribute it to Hawthorn?