Dropping 5.6.0 support

[Bradley Baetz]

The latest DBI release has as a changelog item:

  NOTE: Future versions of the DBI *will not* support perl 5.6.0 or earlier.
  : perl 5.6.1 will be the minimum supported version.

This is something happening in the future, not now, and I don't know of
any specific feature we'd be needing from a newer DBI, but I think tha
this should probably be considered advance warning, or something.
Perhaps we should document this somewhere?

Bradley