GitHub CI: use latest dependencies, add dependabot (#115)

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 0000000..71cdeea
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,10 @@
+# Set update schedule for GitHub Actions
+# See https://docs.github.com/en/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+
+version: 2
+updates:
+
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+    interval: "monthly"
diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index c2798f5..39260f3 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -20,8 +20,8 @@
       matrix:
         sdk: [dev, stable]
     steps:
-      - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v1.2
+      - uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b
+      - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 825cc22..d8f4d13 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -19,11 +19,11 @@
     steps:
       - name: Set up Homebrew
         id: set-up-homebrew
-        uses: Homebrew/actions/setup-homebrew@1bdcefa8e0cb57745fd8ef32c437267dd8aa4222
+        uses: Homebrew/actions/setup-homebrew@29b2973f18e5aca5443f4b3d87f68eda7a27b22b
 
       - name: Cache Homebrew Bundler RubyGems
         id: cache
-        uses: actions/cache@v1
+        uses: actions/cache@ac8075791e805656e71b4ba23325ace9e3421120
         with:
           path: ${{ steps.set-up-homebrew.outputs.gems-path }}
           key: ${{ runner.os }}-rubygems-${{ steps.set-up-homebrew.outputs.gems-hash }}