Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Extensions/MessagesUI/README.markdown

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+# PromiseKit MessagesUI Extensions ![Build Status]
+
+This project adds promises to Apple’s MessagesUI framework.
+
+The Objective-C equivalents of this repo are in the UIKit extensions.
+
+## CocoaPods
+
+```ruby
+pod "PromiseKit/MessagesUI", "~> 6.0"
+```
+
+The extensions are built into `PromiseKit.framework` thus nothing else is needed.
+
+## Carthage
+
+```ruby
+github "PromiseKit/MessagesUI" ~> 3.0
+```
+
+The extensions are built into their own framework:
+
+```swift
+// swift
+import PromiseKit
+import PMKMessagesUI
+```
+
+```objc
+// objc
+@import PromiseKit;
+@import PMKMessagesUI;
+```
+
+
+[Build Status]: https://travis-ci.org/PromiseKit/MessagesUI.svg?branch=master