update the minimum perl version required

On 5.6.2 I got:

t/manifypods_with_utf8.t ....... Undefined subroutine utf8::encode called at t/manifypods_with_utf8.t line 35.

So the stated prereq of 5.6.1 isn't actually true anymore.
Fix MBTest to work with new and old versions of Test::Builder

What MBTest did was never right, but broke completely when TB changed.
This makes it significantly more sane.
Prepare for release 0.4206