Phs/vagrant provision tweaks

Some small things:
* Avoid MR in askari provision, we don't need it.  This helps trim some memory footprint and keeps the script small.  Both the vagrant and travis scripts have been updated.
* Switch to bundler binstubs by moving existing `bin/` -> `exe/`. Aside from the (no longer) overloaded nature of `bin`, there appears to be no downside.