Add actions job to notice outdated dependencies

Author: dtolnay

.github/workflows/ci.yml

@@ -53,3 +53,11 @@ jobs:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@clippy
       - run: cargo clippy --tests -- -Dclippy::all -Dclippy::pedantic
+
+  outdated:
+    name: Outdated
+    runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request'
+    steps:
+      - uses: actions/checkout@v2
+      - run: cargo outdated --exit-code 1