Unpin version of iso8601 package

The python-neutronclient package requires a newer version
of iso8601, so unpin our version to allow the right version
to be installed.

Change-Id: I2f8b1a4b0602df6c7c435700653e21cfc63527ef
Merge pull request #101 from dhellmann/iso8601-version

Unpin version of iso8601 package