Closes bug 1064411: upgrade VM to 14.04; make guest additions update run on every 'vagrant up'

Adding explicit URL to box for older Vagrant versions, e.g. ci.mozilla.org

Only use the base Vagrant image from Ubuntu
Adding some vbguest plugin detection and removing extra --provision call
tidying up locales generation
Use the canonical Vagrant Cloud name for the trusty64 box.

See "ubuntu/trusty64" name at https://vagrantcloud.com/ubuntu/boxes/trusty64 and the "self-hosted" link at https://vagrantcloud.com/ubuntu/boxes/trusty64/versions.
Merge update of Vagrant box to 14.04 and automatic installation and upgrading of VirtualBox Guest Additions. Fix bug 1064411. Close #326.