fix(deisctl): Use $HOME/.deis/units, don't expand tilde (~) shortcut

The go func `user.Current()` returns error now that we build as a
static binary. Don't bother expanding ~ to $HOME, just use $HOME.
I thought about that, but then it seems like we'd have to document *that* behavior as well, and just not doing that seemed simpler to explain. But I can do that if you and other folks think it would be helpful.