Use git clean to delete stale files from an earlier checkout. These
files interfered with various things (specifically, a comment vote test
failed because there was an old comment_votes/create.json.erb template
hanging around that changed the behavior of the POST /comment_votes endpoint).
Upgrade procedure:
$ echo 2.6.3 > .ruby-version
# upgrade ruby-build to know about the latest ruby versions.
# see also: github.com/rkh/rbenv-update
$ cd ~/.rbenv/plugins/ruby-build && git pull
# install the version of ruby specified in .ruby-version.
# see also: github.com/capistrano/rbenv/issues/83
$ rbenv install --skip-existing
# update the shell to use the latest version of ruby by default.
$ rbenv global 2.6.3
# update rubygems and bundler
$ gem update --system
# update the lockfile to use the latest version of bundler
$ bundle update --bundler
