fix(deisctl): refresh-units uses $HOME/.deis/units by default

Changes `deisctl refresh-units` to install to $HOME/.deis/units by default. For a "global" unit install, use `deisctl refresh-units --path=/var/lib/deis/units".

In conjunction with deis/deis.io#39, also begins versioning for `deisctl` in user-data.

Fixes #1957.