Fix bug in virtualenv construction

The quotes in --prompt="[aspen] " were getting escaped by fabricate and
were bleeding through into the generated bin/activate script in the
virtualenv. I don't know why I didn't notice this before today. My best
guess at this point is that I simply wasn't running `make dev` but only
`make build`, which doesn't create a virtualenv.

See also: https://botbot.me/freenode/aspen/msg/3742794/