travis: switch default dist to bionic

Author: scop

.travis.yml

@@ -1,8 +1,6 @@
 language: python
 python:
-  - "2.6"
   - "2.7"
-  - "3.4"
   - "3.5"
   - "3.6"
   - "nightly"
@@ -13,5 +11,10 @@ script:
 matrix:
   allow_failures:
   - python: "nightly"
+  include:
+    - python: "2.6"
+      dist: trusty
+    - python: "3.4"
+      dist: xenial
 
-dist: trusty
+dist: bionic