Update archive.php

Revising the "primary" element to use a "div" vs. "section" to bring it into sync with index.php and single.php
_s: Improve menu toggle accessibility.

Makes it a little easier to use the primary navigation with assistive
devices.

See #545.
Merge pull request #643 from Automattic/aria-toggle

Improve menu toggle accessibility.

Fixes #575.
Merge pull request #580 from jgclarke/patch-1

Use `div`s consistently throughout templates.
_s: Introduce 4.1 archive template tags.

Now uses the new archive template tags and makes archive template
titling way simpler!

Added shims for backwards compatibility, which can be removed once
WordPress 4.3 was released.

See https://core.trac.wordpress.org/changeset/30223
Closes #556.