Restore Swift 3.1 compatibility

We don't actually need Swift 4 for the new `KeyPath` APIs.

PMKVObserver.xcodeproj/project.pbxproj

@@ -372,7 +372,7 @@
 				SKIP_INSTALL = YES;
 				SUPPORTED_PLATFORMS = "iphonesimulator iphoneos macosx watchsimulator watchos appletvsimulator appletvos";
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 3.0;
 			};
 			name = Debug;
 		};
@@ -391,7 +391,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SKIP_INSTALL = YES;
 				SUPPORTED_PLATFORMS = "iphonesimulator iphoneos macosx watchsimulator watchos appletvsimulator appletvos";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 3.0;
 			};
 			name = Release;
 		};
@@ -407,7 +407,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SUPPORTED_PLATFORMS = "macosx iphonesimulator iphoneos appletvsimulator appletvos";
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 3.0;
 			};
 			name = Debug;
 		};
@@ -422,7 +422,7 @@
 				PRODUCT_BUNDLE_IDENTIFIER = com.postmates.PMKVObserverTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SUPPORTED_PLATFORMS = "macosx iphonesimulator iphoneos appletvsimulator appletvos";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 3.0;
 			};
 			name = Release;
 		};

PMKVObserver/KVObserver.swift

@@ -17,6 +17,7 @@ import Foundation
 public typealias KVObserver = PMKVObserver
 
 extension KVObserver {
+    #if swift(>=3.2)
     /// Establishes a KVO relationship to `object`. The KVO will be active until `object` deallocates or
     /// until the `cancel()` method is invoked.
     public convenience init<Object: AnyObject, Value>(object: Object, keyPath: KeyPath<Object,Value>, options: NSKeyValueObservingOptions = [], block: @escaping (_ object: Object, _ change: Change<Value>, _ kvo: KVObserver) -> Void) {
@@ -40,6 +41,7 @@ extension KVObserver {
             block(unsafeDowncast(observer as AnyObject, to: T.self), unsafeDowncast(object as AnyObject, to: Object.self), Change(rawDict: change), kvo)
         })
     }
+    #endif
 
     /// Establishes a KVO relationship to `object`. The KVO will be active until `object` deallocates or
     /// until the `cancel()` method is invoked.

PMKVObserverTests/KVObserverTests.swift

@@ -75,6 +75,7 @@ class KVObserverTests: XCTestCase {
         fired = false
     }
 
+    #if swift(>=3.2)
     func testSwift4KVO() {
         var fired = false
         var token: KVObserver!
@@ -120,6 +121,7 @@ class KVObserverTests: XCTestCase {
         token.cancel()
         fired = false
     }
+    #endif
 
     func testInitialCancel() {
         var fired = false

README.md

@@ -90,6 +90,8 @@ Installing as a framework requires a minimum of iOS 8, OS X 10.9, watchOS 2.0, o
 
 If you install by copying the source into your project, it should work on iOS 7 or later (iOS 6 if you remove KVObserver.swift), and OS X 10.7 or later. Please note that it has not been tested on these versions.
 
+PMKVObserver will compile with Xcode 8.3, but the new `KeyPath` APIs require Xcode 9 or later.
+
 ## Installation
 
 After installing with any mechanism, you can use this by adding `import PMKVObserver` (Swift) or `@import PMKVObserver;` (Objective-C) to your code.
@@ -102,12 +104,6 @@ To install using [Carthage][], add the following to your Cartfile:
 github "postmates/PMKVObserver" ~> 3.0
 ```
 
-This version supports Swift 4.0. For Swift 3, use the following instead:
-
-```
-github "postmates/PMKVObserver" ~> 2.0
-```
-
 ### CocoaPods
 
 To install using [CocoaPods][], add the following to your Podfile:
@@ -116,12 +112,6 @@ To install using [CocoaPods][], add the following to your Podfile:
 pod 'PMKVObserver', '~> 3.0'
 ```
 
-This release supports Swift 4. If you want Swift 3 support, you can use
-
-```
-pod 'PMKVObserver', '~> 2.0'
-```
-
 [CocoaPods]: https://cocoapods.org
 
 ### Manual Installation
@@ -142,6 +132,10 @@ Unless you explicitly state otherwise, any contribution intentionally submitted
 
 ## Version History
 
+#### Development
+
+* Set Swift version back to Swift 3.2. The new `KeyPath` stuff is usable from 3.2 so there's no reason to require 4. PMVKObserver will still compile with Swift 3.1, but the `KeyPath` APIs won't be available unless compiling with Swift 3.2.
+
 #### v3.0.0 (2017-09-12)
 
 * Convert to Swift 4.