Fix CI build

.travis.yml

@@ -19,8 +19,8 @@ before_install:
   - "sudo systemctl --no-pager status elasticsearch"
 
 install:
-  - "pip install --upgrade pip tox six"
-  - "pip install -r requirements.txt"
+  - "pip install --upgrade pip"
+  - "pip install -U -r requirements.txt"
 
 # commands to run samples
 script: