Bump up to 1.6.0

Version 1.5.1.1

Relesae v1.5.1

This is identical to 1.5.0. The story is that we yanked 1.5.0 a few
minutes after releasing it because of some crashes I'd seen, but it
turns out the crashes were unrelated and the changes are good. So unyank
Syck 1.5.0 by republishing it as 1.5.1.

Version 1.5.0

Version 1.4.1

Bump version to 1.4.0.

Version 1.3.0

Version 1.2.0

Version 1.1.0

Tagging v1.0.4.