update python test versions

Author: tianhao64

.travis.yml

@@ -1,12 +1,11 @@
 language: python
 python:
-  - "2.7.13"
-  - "3.6"
+  - "2.7.16"
+  - "3.7.3"
 # command to install dependencies
 install: 
   - pip install -r requirements.txt --extra-index-url file://$PWD/lib --upgrade  --ignore-installed six
   - pip install -r test-requirements.txt
 # command to run tests
 script: pycodestyle samples tests
 script: pytest
-