Bump minor version and use NAPALM 3

.travis.yml

@@ -1,8 +1,8 @@
 language: python
 python:
-- 2.7
-- 3.4
-- 3.5
+- 3.6
+- 3.7
+- 3.8
 install:
 - pip install tox-travis
 - pip install coveralls