Temporarily lock ably-cocoa version

The upcoming ably-cocoa 1.2.37 will revert a change on which the Chat
SDK depends. It will not be possible to make a single version of the
Chat SDK work with ably-cocoa versions 1.2.36 and 1.2.37.

So, in order to make sure that the Chat SDK continues to work once
ably-cocoa 1.2.37 is released, let's temporarily lock the ably-cocoa
dependency to 1.2.36, and release a new version of the Chat SDK. Then,
once ably-cocoa 1.2.37 is released, we can release _another_ version of
the Chat SDK that requires 1.2.37 and above.

Package.swift

@@ -20,7 +20,7 @@ let package = Package(
     dependencies: [
         .package(
             url: "https://github.com/ably/ably-cocoa",
-            from: "1.2.36"
+            exact: "1.2.36"
         ),
         .package(
             url: "https://github.com/apple/swift-argument-parser",