Rebuild the Sass site whenever a new version is released

[nex3]

The Sass site lists the most recent version of LibSass, so to ensure it stays up-to-date we should force a rebuild whenever a new LibSass release is cut. The easiest way to do this is probably to add a Travis CI build stage that runs whenever a new tag is pushed and pokes the Heroku deploy API.

See also sass/dart-sass#271 and sass/sass#2495.