Only install GDAL depedencies for x.y_with_system_site_packages

 * Move GDAL install into a bash script so it can be called as a one-liner inside an if-statement. 

 * Only install GDAL for 2.7_with_system_site_packages and 3.2_with_system_site_packages. Shaves about a minute off build time. (Could go a bit further and only install python-gdal for 2.x and python3-gdal for 3.x, but will only save about 3s.)

 * Add `travis_retry` to `pip install`s, which will try three times, because I saw one time-out.