Update requirements-dev.txt (#890)

Satisfy security alert: https://github.com/NeurodataWithoutBorders/pynwb/network/alert/requirements.txt/urllib3/open

Author: nicain

requirements-dev.txt

@@ -16,5 +16,5 @@ six==1.11.0
 tox==3.0.0
 traceback2==1.4.0
 unittest2==1.1.0
-urllib3==1.23
+urllib3==1.24.2
 virtualenv==15.2.0