chore: switch from Travis to Github Actions for validate (#217)

.github/workflows/validate.yml

@@ -0,0 +1,11 @@
+name: Validate
+on: [push, pull_request]
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.x'
+      - run: python validate.py

.travis.yml

@@ -1,9 +0,0 @@
-sudo: false
-
-language: python
-
-python:
-  - "3.6"
-
-script:
-  - python validate.py