Use SPM badge that is more consistent with other badges

Readme.markdown

@@ -3,7 +3,7 @@
 [![Version](https://img.shields.io/github/release/venmo/DVR.svg)](https://github.com/venmo/DVR/releases)
 ![Swift Version](https://img.shields.io/badge/swift-4-orange.svg)
 [![Carthage compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
-[![Swift Package Manager](https://rawgit.com/jlyonsmith/artwork/master/SwiftPackageManager/swiftpackagemanager-compatible.svg)](https://swift.org/package-manager/)
+[![Swift Package Manager](https://img.shields.io/badge/SPM-compatible-brightgreen.svg)](https://swift.org/package-manager/)
 
 DVR is a simple Swift framework for making fake `NSURLSession` requests for iOS,
 watchOS, and OS X based on [VCR](https://github.com/vcr/vcr).