Add 'org.springframework.geode:spring-geode-bom' (Maven BOM) to project.

The Maven BOM (Bill of Materials) will provide a simpler and consistent SBDG module version dependency management experience for users starting at start.spring.io, who have added the 'Spring for Apache Geode' dependency to build Spring Boot applications using Apache Geode.

Resolves gh-93.

spring-geode-bom/spring-geode-bom.gradle

@@ -0,0 +1,14 @@
+import io.spring.gradle.convention.SpringMavenPlugin
+
+apply plugin: 'io.spring.convention.bom'
+apply from: MAVEN_POM_EDITOR_GRADLE
+
+sonarqube.skipProject = true
+
+project.rootProject.allprojects.each { p ->
+    p.plugins.withType(SpringMavenPlugin) {
+        if (!project.name.equals(p.name)) {
+            project.mavenBom.projects.add(p)
+        }
+    }
+}