ci: upgrade to Go 1.17 and use cache now that go.sum exists

- this should fix the failing CI on this PR
  - this PR was made before my Travis -> GH Actions migration PR was merged
  - now that it's merged, I've rebased this PR on top and made tiny adjustments

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        go-version: ["1.14"]
+        go-version: ["1.17"]
         os: [ubuntu-latest, macOS-latest]
 
     steps:
@@ -22,7 +22,7 @@ jobs:
       uses: actions/setup-go@v3
       with:
         go-version: ${{ matrix.go-version }}
-        # cache: true # TODO: cache needs path to go.sum per https://github.com/actions/setup-go#caching-dependency-files-and-build-outputs
+        cache: true
     - name: Install
       run: |
         bash --version ; type bash