Freeze iso8601 for older stable

As explained in Iece0116d3c412216251e2d7fbffe10bfe2c1c789
master bumped iso8601 version requirement to 0.1.8
but we can't do that on older stable branches, so cap the version instead.

Change-Id: I5677a294a1428774fc5c3b767bd4a7c8567d02ff