slaprunner: version up: gunicorn 19.10.0

This change facilitates the creation of a new software release for
Python 3.

futures is a backport of Py3's concurrent.futures module.

By upgrading gunicorn to version 19.10.0 (which has:

if sys.version_info[0] < 3:
    extra_require['gthread'] = ['futures']

), we won't need to remove the "futures" egg from software-py3.cfg,
i.e.:

eggs -=
  futures

software/slaprunner/software.cfg

@@ -151,8 +151,7 @@ eggs =
   collective.recipe.shelloutput
   slapos.recipe.build
   slapos.toolbox[flask_auth]
-  gunicorn==19.7.1
-  futures
+  gunicorn[gthread]
   ${slapos-cookbook:eggs}
   slapos.core # listed explicitly for scripts generation
 
@@ -167,7 +166,7 @@ Flask-Auth = 0.85
 cns.recipe.symlink = 0.2.3
 futures = 3.0.5
 gitdb = 0.6.4
-gunicorn = 19.7.1
+gunicorn = 19.10.0
 prettytable = 0.7.2
 pycurl = 7.43.0
 slapos.recipe.template = 4.4