[readme] Remove downloads badge

Currently there are no downloads so this should be re-added when there
actually are.

README.md

@@ -5,7 +5,6 @@
 <img src="https://img.shields.io/badge/language-swift-orange.svg" alt="Language: Swift">
 <a href="https://cocoapods.org/pods/Unreachable"><img src="https://img.shields.io/cocoapods/v/Unreachable.svg" alt="CocoaPods - Unreachable"></a>
 <a href="https://github.com/Carthage/Carthage"><img src="https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat" alt="Carthage"></a>
-<img src="https://img.shields.io/cocoapods/dt/Unreachable.svg" alt="downloads">
 <a href="https://codebeat.co/projects/github-com-nvzqz-unreachable-master"><img src="https://codebeat.co/badges/3e01a261-54d1-400e-8820-6aadb177c01a" alt="codebeat badge"></a>
 <img src="https://img.shields.io/badge/license-MIT-000000.svg" alt="License">
 </p>