Bumped CI to Swift 5.7, removed 5.5 and 5.6 (#71)

* Bumped CI to Swift 5.7, removed 5.5 and 5.6

* Triggering CI

Author: bdrelling

.github/workflows/tests.yml

@@ -14,7 +14,7 @@ jobs:
     with:
       scheme: AudioKitUI
       platforms: iOS macOS
-      swift-versions: 5.5 5.6
+      swift-versions: 5.7
 
   # Send notification to Discord on failure.
   send_notification: