travis: remove py2.6 add py3.7

.travis.yml

@@ -1,7 +1,6 @@
 language: python
 sudo: false
 python:
-    - "2.6"
     - "2.7"
     - "3.3"
     - "3.4"
@@ -10,6 +9,11 @@ python:
     - "nightly"
     - "pypy"
     - "pypy3"
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 install:
     - make init
     - pip install coveralls