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 `open-release/hawthorn.master` 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? If not, let me know and I will make the same PR to `master` branch.