expend version constraint on mixlib-shellout

the change for mixlib-shellout 2.0 drops the forcing of LC_ALL=C
which breaks UTF-8 encoding, so this is probably a good thing for
test-kitchen, but in the absense of any bug reports about encoding
problems with test-kitchen the mixlib-shellout 1.x branch still
works fine for most users so there's no need to raise the floor.