* upgrade vagrant box to xenial
this allows the redis version to be upgraded to support the new redis
features used in the activity tracker
* add libpam0g package to vagrant box
this is required for native extensions of gems to build after the
addition of PAM support was added in #5303
RVM has a known issue with installing Ruby 2.5 on the version of Ubuntu
the Vagrant box is using: https://github.com/rvm/rvm/issues/4291
This bug was preventing any gem installs in the vagrant box
Daniel King