Remove requirement for Swift 5.3 (#823)

* Fix requirement for Swift 5.3

Make the code compatible with Swift 5.1 and 5.2 again. Resolves #822

* Added entry to Changelog

CHANGELOG.md

@@ -1,4 +1,7 @@
 # master
+
+1. Fixed issue with `SignalProducer.Type.interval()` making Swift 5.3 a requirement. (#823 kudos to @mluisbrown) 
+
 *Please add new entries at the top.*
 
 # 6.6.0

Sources/SignalProducer.swift

@@ -3081,7 +3081,9 @@ extension SignalProducer where Error == Never {
 			)
 		}
 	}
+}
 
+extension SignalProducer where Error == Never, Value == Int {
 	/// Creates a producer that will send the sequence of all integers
 	/// from 0 to infinity, or until disposed.
 	///
@@ -3098,7 +3100,7 @@ extension SignalProducer where Error == Never {
 	public static func interval(
 		_ interval: DispatchTimeInterval,
 		on scheduler: DateScheduler
-	) -> SignalProducer where Value == Int {
+	) -> SignalProducer {
 		.interval(0..., interval: interval, on: scheduler)
 	}
 }