Fix #4131: Upgrade to Ruby 2.6.

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

config/deploy/production.rb

@@ -1,6 +1,5 @@
 set :user, "danbooru"
 set :rails_env, "production"
-set :rbenv_path, "/home/danbooru/.rbenv"
 append :linked_files, ".env.production"
 
 server "kagamihara", :roles => %w(web app), :primary => true