update-wirbelsturm.sh: update Vagrant plugins
Reformat code
vagrant-hosts: require at least version 2.1.4
Require Vagrant version >= 1.6.1
WS-10: source rvm script after ruby-bootstrap

This ensures we can e.g. run librarian-puppet in later bootstrap stages
in the case this is a fresh installation of Ruby/rvm on a host box.
Use short names for referring to GitHub tickets
CHANGELOG: we require vagrant-hosts 2.1.4 or later