update travis-ci file

Signed-off-by: Tianhao He <[email protected]>

Author: tianhao64

.travis.yml

@@ -4,7 +4,7 @@ python:
   - "3.6"
 # command to install dependencies
 install: 
-  - pip install -r requirements.txt
+  - pip install -r requirements.txt --upgrade --ignore-installed six
   - pip install -r test-requirements.txt
   - pip install pycodestyle
 # command to run tests