Rebuild the Sass site whenever a new version is released

[nex3]

From sass/sass#2495:

The Sass site lists the most recent version of Ruby Sass and (currently) uses Ruby Sass as the source of truth for its reference documentation, so to ensure it stays up-to-date we should force a rebuild whenever a new Ruby Sass 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/libsass#2618.