explicitly specify the distro to get py26 and py33

tomato42 · tomato42 · commit 14abfe020d49 · 2019-10-07T14:47:08.000+02:00
backport of 091ca06
diff --git a/.travis.yml b/.travis.yml
@@ -1,3 +1,6 @@
+# workaround for 3.7 not available in default configuration
+# travis-ci/travis-ci#9815
+dist: trusty
 language: python
 python:
   - "2.6"
