Update README.md

I got this error on Cocoapods 1.0.0.
https://github.com/ashleymills/Reachability.swift/issues/73

Fix README.md

> pod 'ReachabilitySwift', :git => 'https://github.com/ashleymills/Reachability.swift'
< pod 'ReachabilitySwift'

Thanks.

README.md

@@ -27,7 +27,7 @@ Just drop the **Reachability.swift** file into your project. That's it!
 
     ``` ruby
     use_frameworks!
-    pod 'ReachabilitySwift', :git => 'https://github.com/ashleymills/Reachability.swift'
+    pod 'ReachabilitySwift'
     ```
 
  3. Run `pod install`.