slapos-testing: pin urllib3 to fix build

  Traceback of the tests:

    Installing eggs.
While:
  Installing eggs.
  Getting distribution for 'urllib3>=1.18'.
Error: Picked: urllib3 = 1.24.1

software/slapos-testing/software.cfg

@@ -199,7 +199,7 @@ pycurl = 7.43.0.2
 pyflakes = 2.0.0
 smmap2 = 2.0.4
 zope.testing = 4.6.2
-
+urllib3 = 1.24.1 
 # Required by:
 # caucase
 PyJWT = 1.6.4