Upgrade Scala to 2.12

See #1058

- recommended version in the 2.2 documentation.

Author: garyrussell

build.gradle

@@ -81,7 +81,7 @@ subprojects { subproject ->
 		kafkaVersion = '2.2.0'
 		log4jVersion = '2.11.2'
 		mockitoVersion = '2.24.0'
-		scalaVersion = '2.11'
+		scalaVersion = '2.12'
 		springRetryVersion = '1.2.4.RELEASE'
 		springVersion = '5.2.0.BUILD-SNAPSHOT'
 		springDataCommonsVersion = '2.2.0.BUILD-SNAPSHOT'

src/reference/asciidoc/appendix.adoc

@@ -36,14 +36,14 @@ When you use `spring-kafka-test` (version 2.2.x) with the 2.1.x `kafka-clients`
 
 <dependency>
     <groupId>org.apache.kafka</groupId>
-    <artifactId>kafka_2.11</artifactId>
+    <artifactId>kafka_2.12</artifactId>
     <version>2.1.0</version>
     <scope>test</scope>
 </dependency>
 
 <dependency>
     <groupId>org.apache.kafka</groupId>
-    <artifactId>kafka_2.11</artifactId>
+    <artifactId>kafka_2.12</artifactId>
     <version>2.1.0</version>
     <classifier>test</classifier>
     <scope>test</scope>