Replace Git with HTTPS URL for Twerl dep

Running `make` on the release-0.12.x branch (and 0.12.2) tag fails with a ‘Host key verification failed’ warning on machines that have not yet added GitHub as a known host (e.g., production or newly set-up developer instances). This is because the Twerl dependency has a SSH URL. GitHub [recommends cloning vendors over HTTPS] (https://help.github.com/articles/which-remote-url-should-i-use/#cloning-with-https-recommended) instead.