Add size option to tmpfs mount command.
Remove duplicate gems from commander gemfile.
Error out when root perms are unobtainable.

Earlier we would warn and proceed - which is dumb, because all subsequent
ops need super perms.
Make tmpfs mount/unmount a common routine.

We often need to perform some actions on a mounted tmpfs. This involves
code duplication for setup/cleanup of the tmpfs mountpoint. So we extract
this into a common method that can yield to a block of code that needs such
a mountpoint, and cleanup after it.