Migrate to GitHub Actions.

.github/workflows/main.yml

@@ -0,0 +1,32 @@
+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-nursery/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
+    - name: Check for unstable features
+      run: (cd stable-check && cargo run -- ../src)
+    - name: Check for broken links
+      run: tests/linkcheck.sh