[travis] Force dist to Ubuntu Trusty by default

According to https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment Ubuntu Xenial is now default, but it lacks python 2.6, 3.2 and 3.3 support needed by tests

.travis.yml

@@ -9,6 +9,7 @@ python:
   - "3.6"
   - "pypy"
   - "pypy3"
+dist: trusty
 env:
   - YTDL_TEST_SET=core
   - YTDL_TEST_SET=download