slaprunner: downgrade gunicorn to v19.4.5 because of bug in later versions

In gunicorn v19.6 and v19.5, a bug was introduced which deletes socket when nginx worker exits.
In our case, it crashes the slaprunner and prevents it from restarting by slapos.
Let's stick to v19.4.5 until this issue is closed : https://github.com/benoitc/gunicorn/issues/1298

software/slaprunner/common.cfg

@@ -218,7 +218,7 @@ eggs =
   plone.recipe.command
   slapos.recipe.build
   slapos.toolbox[flask_auth]
-  Gunicorn
+  gunicorn
   futures
   ${slapos-cookbook:eggs}
 

software/slaprunner/software.cfg

@@ -13,7 +13,7 @@ cns.recipe.symlink = 0.2.3
 collective.recipe.environment = 0.2.0
 futures = 3.0.5
 gitdb = 0.6.4
-gunicorn = 19.6.0
+gunicorn = 19.4.5
 prettytable = 0.7.2
 pycurl = 7.43.0
 slapos.recipe.template = 2.9