Update requirements.rst

Installing maec==4.0.1.0 fails, because it is apprently incompatible with the most recent cybox version. The installation just succeeds when explicitly specifying cybox==2.0.1.4.

Further reference: https://github.com/MAECProject/python-maec/issues/25
Merge pull request #380 from bombadil/patch-1

Update requirements.rst