ci: fix travis build config validation messages

Build config validation:
- root: deprecated key sudo (The key `sudo` has no effect anymore.)
- deploy.on: unknown key distributions (sdist bdist_wheel)
- root: missing os, using the default linux
- deploy: key user is an alias for username, using username

from https://travis-ci.org/github/dhimmel/obonet/jobs/730683465/config

.travis.yml

@@ -1,5 +1,5 @@
+os: linux
 dist: focal
-sudo: false
 language: python
 python:
   - "3.5"
@@ -17,11 +17,11 @@ script:
   - pytest --color=yes
 deploy:
   - provider: pypi
-    user: __token__
+    username: __token__
     password: $pypi_password
     on:
       python: "3.8"
       condition: $NETWORKX_VERSION = 2.*
       repo: dhimmel/obonet
       tags: true
-      distributions: sdist bdist_wheel
+    distributions: sdist bdist_wheel