ci(travis): clear Build config validation items & general refresh

* https://travis-ci.org/github/poljar/weechat-matrix/builds/685282618/config

```
Build config validation

root: deprecated key sudo (The key `sudo` has no effect anymore.)
root: missing os, using the default linux
```

.travis.yml

@@ -1,6 +1,12 @@
-language: python
-dist: xenial
-sudo: false
+---
+## Machine config
+os: "linux"
+arch: "amd64"
+dist: "bionic"
+version: "~> 1.0"
+
+## Language config
+language: "python"
 python:
   - "3.6"
   - "3.7"
@@ -17,5 +23,4 @@ install:
   - pip install pytest
   - pip install hypothesis
 
-script:
-    python -m pytest
+script: python -m pytest