Test against swift 5.5, 5.6, 5.7 (#10)

* Test against swift 5.5,5.6,5.7

* Update swift.yml

Author: adam-fowler

.github/workflows/swift.yml

@@ -1,6 +1,15 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+    - main
+  pull_request:
+    branches:
+    - main
+  release:
+    types: [published]
+  workflow_dispatch:
 
 jobs:
   macOS:
@@ -18,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        image: ['swift:5.1', 'swift:5.2', 'swift:5.3', 'swift:5.4']
+        image: ['swift:5.1', 'swift:5.4', 'swift:5.5', 'swift:5.6', 'swift:5.7']
     container:
       image: ${{ matrix.image }}
     steps: