Remove Spring IO Platform Dependency Management.

Declare Spring's Dependency Management Gradle Plugin to manage project dependency versions.

docs/spring-session-docs.gradle

@@ -1,8 +1,16 @@
+import org.springframework.boot.gradle.plugin.SpringBootPlugin
+
 apply plugin: 'io.spring.convention.docs'
 apply plugin: 'io.spring.convention.spring-test'
-
+apply plugin: 'io.spring.dependency-management'
 apply from: IDE_GRADLE
 
+dependencyManagement {
+	imports {
+		mavenBom SpringBootPlugin.BOM_COORDINATES
+	}
+}
+
 dependencies {
 
 	testCompile project(':spring-session-data-geode')