Build the stable version of pkgdown site automatically

[yutannihilation]

With the current setting, Travis deploys only when the branch is master, so the tagging the release on a non-master branch (which we did in v3.1.0, v3.2.0 and v3.2.1) won't invoke the pkgdown build. Of course, we can manually invoke the build by pushing an tweaked branch to this repo (e.g. c616035), but I hope this will be done automatically...

Probably, we can change the Travis setting to deploy when the branch is master OR the event is tagging.

https://docs.travis-ci.com/user/deployment/#conditional-releases-with-on