Adding Podspec for Swift 4 support (#158)

* Bumping podspec

This is to incorporate the Swift 4 support that was merged into master.

* Updating podspec to 5.0.0

Since the Swift 4 upgrade was a breaking change, we're bumping this to 5.0.0 instead of 4.2.0.

* Updating README to reflect Swift 4/ImageViewer 5

ImageViewer.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
     s.name             = "ImageViewer"
-    s.version          = "4.1.0"
+    s.version          = "5.0.0"
     s.summary          = "An image viewer à la Twitter"
     s.description  = <<-EOS
    ImageViewer is a library that enables a user to visualize an image in fullscreen. Besides the typical pinch and double tap to zoom, we also provide a vertical swipe to dismiss. Finally, we try to mimic the displacement of the image from its current container into fullscreen, this feature being its main selling point. We also offer an amazing Gallery, so you can swipe between images.

README.md

@@ -1,6 +1,6 @@
 
 [![CI Status](http://img.shields.io/travis/MailOnline/ImageViewer.svg?style=flat)](https://travis-ci.org/MailOnline/ImageViewer)
-[![Swift 3.1](https://img.shields.io/badge/Swift-3.1-orange.svg?style=flat)](https://developer.apple.com/swift/)
+[![Swift 3.1](https://img.shields.io/badge/Swift-4.0-orange.svg?style=flat)](https://developer.apple.com/swift/)
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 [![Version](https://img.shields.io/cocoapods/v/ImageViewer.svg?style=flat)](http://cocoadocs.org/docsets/ImageViewer)
 [![Platforms iOS](https://img.shields.io/badge/Platforms-iOS-lightgray.svg?style=flat)](https://developer.apple.com/swift/)
@@ -49,13 +49,14 @@ extension ViewController: GalleryItemsDataSource {
 
 ### ImageViewer version vs Swift version.
 
-ImageViewer 4.0+ is Swift 3 ready! 🎉
+ImageViewer 5.0+ is Swift 4 ready! 🎉
 
 If you use earlier version of Swift - refer to the table below:
 
 | Swift version | ImageViewer version               |
 | ------------- | --------------------------------- |
-| 3.x           | >= 4.0                            |
+| 4.x           | >= 5.0                            |
+| 3.x           | 4.0                               |
 | 2.3           | 3.1 [⚠️](CHANGELOG.md#version-31) |
 | 2.2           | <= 2.1                            |