Add mutual links to D7 versions

[ptmkenny]

Thanks for this example; it's very helpful.

To aid discoverability, I think it would be great if the README linked to the D7 examples as well:

https://github.com/pantheon-systems/example-drupal7-circle-composer
https://github.com/pantheon-systems/example-drupal7-travis-composer

Google gave me the Drupal 7 version but the D8 version was buried in the results; it took me a bit of time to find it, but I could have spotted it easily if it was in the README.