Added failing test for #756
Don't clobber latest_revision_created_at

Fixes #756

This field was being set to None whenever a revision was pushed live
Merge branch 'issue-756' of https://github.com/kaedroho/wagtail into kaedroho-issue-756

Conflicts:
	wagtail/wagtailcore/tests/test_page_model.py
release note for #767
Merge branch 'kaedroho-issue-756'
Merge branch 'master' of github.com:torchbox/wagtail