install-stack step in INSTALL.md fails with "No such file or directory"

[edmorley]

Following the cedar-14 steps in INSTALL.md on master, I get:

...
$ sudo /vagrant/bin/install-stack /tmp/cedar64-14.4.0.img.gz

-----> Starting install at Mon Jul 3 13:55:15 UTC 2017

-----> Gunzipping image /tmp/cedar64-14.4.0.img.gz
/vagrant/bin/install-stack: line 16: /mnt/stacks/cedar64-14.4.0.img: No such file or directory

This is because /mnt/stacks doesn't exist in a clean Vagrant instance.

I guess install-stack should just do a mkdir -p /mnt/stacks before the gzip, just in case it's being run from somewhere that doesn't already have existing images installed.