Unpin iso8601 but skip bad versions

0.1.7 is now released (https://pypi.python.org/pypi/iso8601). We
do need to allow folks to use the old 0.1.4 or the new  0.1.7 and
avoid the two bad versions (0.1.5 and 0.1.6)

Change-Id: Ic0784ec0a0d380da1b745a8c335fa4edb865fad8