.travis.yml: Update Python versions and remove 2.6

Python 2.6 is not among supported Python anymore
https://docs.travis-ci.com/user/languages/python/

Author: techtonik

.travis.yml

@@ -1,12 +1,13 @@
 language: python
 python:
-  - "2.6"
   - "2.7"
   - "3.3"
   - "3.4"
   - "3.5"
-  - "3.5-dev" # 3.5 development branch
-  - "nightly" # currently points to 3.6-dev
+  - "3.6"
+  - "3.7"
+  - "3.7-dev"
+  - "3.8-dev"
 
 install:
   - pip install coverage