deps: Specify the minimum version required for blessings

Buildstream uses the Terminal.does_styling attribute, which was only
added in blessings 1.6.

Blessings helpfully returns an empty string when calling an nonexistent
attribute on the Terminal object, instead of failing.

Because Fedora has blessings 1.5, Buildstream thought that my terminal
couldn't handle styling, and just didn't print the status bar, silently.

Specifying the minimum version avoids this silent failure.