Port from Travis to GitHub Actions

Based on rust-lang/reference#705; thanks @ehuss!

.github/workflows/main.yml

@@ -0,0 +1,28 @@
+name: CI
+on: [push, pull_request]
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: Update rustup
+      run: rustup self update
+    - name: Install Rust
+      run: |
+        rustup set profile minimal
+        rustup toolchain install nightly -c rust-docs
+        rustup default nightly 
+    - name: Install mdbook
+      run: |
+        mkdir bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.4/mdbook-v0.3.4-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        echo "##[add-path]$(pwd)/bin"
+    - name: Report versions
+      run: |
+        rustup --version
+        rustc -Vv
+        mdbook --version
+    - name: Run tests
+      run: mdbook test