set hard dependency on peewee2

as generic foreign key has been removed from the playhouse
namespace we have to set dependency to the older peewee version

setup.py

@@ -15,7 +15,7 @@ setup(
     ],
     install_requires=[
         'fqdn',
-        'peewee',
+        'peewee<3',
     ],
     scripts=[
         'bin/ca-server',