Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/PromiseKit.playground/Contents.swift

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+import PlaygroundSupport
+
+// Is this erroring? If so open the `.xcodeproj` and build the
+// framework for a macOS target (usually labeled: “My Mac”).
+// Then select `PromiseKit.playground` from inside Xcode.
+import PromiseKit
+
+
+func promise3() -> Promise<Int> {
+    return after(.seconds(1)).map{ 3 }
+}
+
+firstly {
+    Promise.value(1)
+}.map { _ in
+    2
+}.then { _ in
+    promise3()
+}.done {
+    print($0)  // => 3
+}.catch { error in
+    // only happens for errors
+}.finally {
+    PlaygroundPage.current.finishExecution()
+}
+
+PlaygroundPage.current.needsIndefiniteExecution = true

Carthage/Checkouts/PromiseKit/PromiseKit.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='3.0' sdk='macosx' auto-termination-delay='2'>
+    <sections>
+        <code source-file-name='section-1.swift'/>
+    </sections>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>

Carthage/Checkouts/PromiseKit/PromiseKit.playground/playground.xcworkspace/contents.xcworkspacedata

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+<?xml version="1.0" encoding="UTF-8"?>
+<Workspace
+   version = "1.0">
+   <FileRef
+      location = "self:">
+   </FileRef>
+</Workspace>