updated travis with supported versions

Author: auvipy

.travis.yml

@@ -2,18 +2,16 @@ language: python
 sudo: false
 cache: false
 python:
-    - '3.5'
+    - '3.6'
 env:
   global:
     PYTHONUNBUFFERED=yes
   matrix:
-    - TOXENV=2.6
     - TOXENV=2.7
-    - TOXENV=3.3
-    - TOXENV=3.4
-    - TOXENV=3.4
-    - TOXENV=pypy
+    - TOXENV=3.7
+    - TOXENV=3.6
     - TOXENV=3.5
+    - TOXENV=pypy
     - TOXENV=pypy3
 install: travis_retry pip install -U tox
 script: tox -v -- -v