[vagrant] Make sure graphicsmagick is installed

Without graphicsmagick installed there are errors when running the
tests [1].

Added graphicsmagick to the list of packages installed by apt-get.

[1] https://github.com/mysociety/popit/issues/280#issuecomment-31962704
[vagrant] Tidy up Vagrantfile

Various fixes to ensure the example Vagrantfile works out of the box as
described by the documentation.

- Re-enable the provision.sh script (removed in
  4da43939f46671192ecaba525e0715ea3314c464, presumably by accident
  during the upgrade)
- Make sure code is available at /vagrant/popit
- Use vagrant's default network settings
- Don't use NFS and remove related code