Manage Kotlin version consistently

Closes gh-564

initializr-generator/src/main/java/io/spring/initializr/generator/ProjectGenerator.java

@@ -357,10 +357,6 @@ public class ProjectGenerator {
 		model.put("kotlinSupport", kotlinSupport);
 
 		if (isMavenBuild(request)) {
-			if (kotlinSupport) { // No need to provide a kotlin.version
-				request.getBuildProperties().getVersions()
-						.remove(new VersionProperty("kotlin.version"));
-			}
 			model.put("mavenBuild", true);
 			ParentPom parentPom = metadata.getConfiguration().getEnv().getMaven()
 					.resolveParentPom(request.getBootVersion());

initializr-generator/src/test/resources/project/maven/kotlin-java9-pom.xml.gen

@@ -23,6 +23,7 @@
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
 		<java.version>9</java.version>
+		<kotlin.version>1.1.1</kotlin.version>
 	</properties>
 
 	<dependencies>

initializr-generator/src/test/resources/project/maven/kotlin-springboot2-pom.xml.gen

@@ -23,6 +23,7 @@
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
 		<java.version>1.8</java.version>
+		<kotlin.version>1.1.1</kotlin.version>
 	</properties>
 
 	<dependencies>