chore: test against supported Go versions

.github/workflows/test.yml

@@ -0,0 +1,17 @@
+name: test
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        version: [ '1.14', '1.15' ]
+    name: Go ${{ matrix.version }}
+    steps:
+    - uses: actions/setup-go@v2
+      with:
+        go-version: ${{ matrix.version }}
+    - uses: actions/checkout@v2
+    - run: ./test.sh

README.md

@@ -1,4 +1,4 @@
-[![CI](https://github.com/cloudfoundry/jsonry/workflows/Go/badge.svg?branch=main)](https://github.com/cloudfoundry/jsonry/actions?query=workflow%3AGo)
+[![test](https://github.com/cloudfoundry/jsonry/workflows/test/badge.svg?branch=main)](https://github.com/cloudfoundry/jsonry/actions?query=workflow%3Atest+branch%3Amain)
 [![go.dev reference](https://img.shields.io/badge/go.dev-reference-007d9c?logo=go&logoColor=white&style=flat-square)](https://pkg.go.dev/code.cloudfoundry.org/jsonry?tab=doc)
 
 # JSONry