Target scala 2.12 in Readme badge & point to latest Akka-http version

Author: aaabramov

README.md

@@ -2,9 +2,9 @@
 
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+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.softwaremill.akka-http-session/core_2.12/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.softwaremill.akka-http-session/core_2.12)
 
-[`akka-http`](http://doc.akka.io/docs/akka/2.4.2/scala/http/index.html) is an Akka 
+[`akka-http`](https://doc.akka.io/docs/akka-http/current/index.html) is an Akka 
 module, originating from [spray.io](http://spray.io), for building *reactive* REST services with an elegant DSL.
 
 `akka-http` is a great toolkit for building backends for single-page or mobile applications. In almost all apps there