Allow pageBreakSource as an unprefixed meta property

[mattgarrish]

The pageBreakSource property has been add to the Meta properties vocabulary in epub 3.4 so the use of the unprefixed property in meta element property attributes shouldn't cause warnings or errors.

[mattgarrish]

Searching through closed issues I see we asked for this for incubation in #1507

Probably all that needs changing is to remove the incubation comment.