Adjust usage of unless in "drop NOT NULL constraints" spec.

The previous form of that spec was a bit difficult (for humans) to
parse.  A further refinement will eliminate the looping.
Upgrade to FakeFS 0.4.2.

FakeFS 0.4.0 is incompatible with recent versions of addressable.

FakeFS::Pathname, for some reason, doesn't seem to participate well in
Ruby's string-coercion protocol (despite defining #to_s), so for now
we're just going to coerce it explicitly.  (It's harmless otherwise.)