Test::More doesn't require a minimum version of Test::Builder

[doherty]

I use Test::More 0.94; because I use subtests, but today I learned (the hard way: doherty/String-Trim#1) that you also need Test::Builder 0.94. Since Test::More doesn't require a minimum version of Test::Builder, it was possible to have a Test::More that knows how to do subtests, but a Test::Builder that doesn't know how to do them.