Bring in vagrant over virtual box.

Provisioning is done with `scripts/vagrant`: I realized salt is
overkill for this, and I don't want the dependency.
Let's see if we can do without this now.
Don't merge coverage results after each run.

This allows simplecov to avoid using a lockfile while attempting the
merge.  This lockfile in turn does not work with synched directories
inside virtual box: the lock silently hangs forever instead of
acquiring.

See also:
* https://gist.github.com/k-yamada/3930916/#comment-953584
* https://github.com/colszowka/simplecov/issues/258