Travis CI: fix some config warnings

`sudo: false` used to be the way to opt in to their container-based
infrastructure. That system was retired in 2018 and so the `sudo` option
no longer does anything. Remove it.

Travi CI supports multiple OSes now. Add `os: linux` to be explicit
about which OS we want to test on.

.travis.yml

@@ -1,3 +1,4 @@
+os: linux
 language: python
 python:
     - "2.7"
@@ -13,4 +14,3 @@ before_script: pokedex setup -v
 script:
     - py.test
     - pokedex dump -l all
-sudo: false