[travis] Remove sudo: false

Travis now recommends removing `sudo: false` from configuration: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration.

.travis.yml

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