Switch from Travis CI to GitHub Actions (#20)

.github/CONTRIBUTING.md

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+# Contributing to Map Tools
+
+Thank you for your interest in Map Tools! Before contributing,
+be sure to know about these few guidelines:
+
+- Contributions have to be licensed under the zlib license (or compatible)
+  for code, and CC BY-SA 3.0 (or compatible) for assets.
+- Make sure to update the changelog, keeping the
+  [changelog format](http://keepachangelog.com/en/1.0.0/) we use.
+- Don't bump the version yourself. Maintainers will do this when necessary.

.github/workflows/build.yml

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+on: [push, pull_request]
+name: build
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: lint
+        uses: Roang-zero1/factorio-mod-luacheck@master
+        with:
+          luacheckrc_url: ""