added version requirement for dateutil

- update to version 0.16.0:
* [WARN] Arrow will drop support for Python 2.7 and 3.5 in the 1.0.0
release in late September. The 0.16.x and 0.17.x releases are the
last to support Python 2.7 and 3.5.
* [NEW] Implemented PEP 495 to handle ambiguous datetimes. This is
achieved by the addition of the fold attribute for Arrow objects.
* [NEW] Added normalize_whitespace flag to arrow.get. This is useful
for parsing log files and/or any files that may contain
inconsistent spacing.