Upgrade bootsnap to 1.9.3 too because Ruby 3.0.3 has a bug that causes
Rails to fail to boot when bootsnap is enabled. Bootsnap 1.9.3 works
around this bug.
Also add libgmp to build with bignum support.
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
