Allow to force a concrete Jekyll version

This change allows to build the site using a concrete Jekyll version.

By default it will keep the current behavior and use the default version (the one reported by the `jekyll --version` command), but now the script can be invoked as follows to force a concrete version of the Jekyll gem:

```bash
JEKYLL=1.5.1 ./deploy-site.sh 
```

This comes in handy when you have several versions of the gem installed and you have to build the site with a concrete one (1.5.1 is the one our CI system uses).