stack/slapos: version ↑ six 1.11.0

jsonschema needs six>=1.11.0

Version and requirements information containing six:
  [versions] constraint on six: 1.10.0
  Requirement of mock==2.0.0: six>=1.9
  Requirement of slapos.core==1.4.11: six
  Requirement of bcrypt==3.1.4: six>=1.4.1
  Requirement of cryptography==2.3.1: six>=1.4.1
  Requirement of jsonschema: six>=1.11.0
  Requirement of manuel: six
  Requirement of cliff: six>=1.9.0
  Requirement of pyOpenSSL>=18.0.0: six>=1.5.2

stack/slapos.cfg

@@ -133,7 +133,7 @@ pyOpenSSL = 18.0.0
 pyparsing = 2.2.0
 pytz = 2016.10
 requests = 2.13.0
-six = 1.10.0
+six = 1.11.0
 slapos.cookbook = 1.0.75
 slapos.core = 1.4.11
 slapos.extension.strip = 0.4