Simplify the workflow file

This uses a matrix configuration to set up all the various
configurations we're testing tracks with. It also adds testing on ruby
3.0 and testing postgres with all the various ruby versions.

.github/workflows/continuous-integration.yml

@@ -1,38 +1,13 @@
 ---
 name: Automated tests
-
 on: pull_request
-
 jobs:
-  test25mysql:
+  build:
     runs-on: ubuntu-latest
+    strategy: matrix
+    matrix:
+      ruby: ["2.5", "2.6", "2.7", "3.0"]
+      db: [sqlite, mysql, postgres]
     steps:
       - uses: actions/checkout@v1
-      - run: bash -x script/cibuild 2.5 mysql
-
-  test26mysql:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - run: bash -x script/cibuild 2.6 mysql
-
-  test27mysql:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - run: bash -x script/cibuild 2.7 mysql
-
-  test27postgres:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - run: bash -x script/cibuild 2.7 postgres
-
-  test27sqlite:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v1
-      - run: bash -x script/cibuild 2.7 sqlite
-
-#      - run: bash -x script/cibuild 2.5 postgres
-#      - run: bash -x script/cibuild 2.7 postgres
+      - run: bash -x script/cibuild ${{ matrix.ruby }} ${{ matrix.db }}