requirements.txt: Add python-dateutil to work around problem with modulemd
requirements.txt: Add python-dateutil to work around problem with modulemd

The 1.3.0 release of modulemd (which is currently on PyPI) is missing
a requirement on python-dateutil. Add it in our requirements.txt for now
to avoid test failures.

vrutkovs/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,fr34k8/atomic-reactor,maxamillion/atomic-reactor,vrutkovs/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor