Apply dependency management to selected modules

This commit applies the Dependency Management Plugin to modules that
require it; right now Spring Framework is importing BOMs for Netty and
Reactor dependencies only.

Instead of applying those BOMs to all modules, they're applied only
where they're needed.

Issue: SPR-15885

spring-websocket/spring-websocket.gradle

@@ -1,5 +1,18 @@
 description = "Spring WebSocket"
 
+apply plugin: "io.spring.dependency-management"
+
+dependencyManagement {
+	imports {
+		mavenBom "io.projectreactor:reactor-bom:${reactorVersion}"
+		mavenBom "io.netty:netty-bom:${nettyVersion}"
+	}
+	resolutionStrategy {
+		cacheChangingModulesFor 0, 'seconds'
+	}
+	applyMavenExclusions = false
+}
+
 dependencies {
 	compile(project(":spring-context"))
 	compile(project(":spring-core"))