Upgrade Gradle plugin’s ID so it’s suitable for the Plugin Portal

Gradle’s plugin portal requires each plugin’s ID to be in a namespace.
Our existing ID, spring-boot, does not meet this requirement. This
commit changes the plugin’s ID to org.springframework.boot.spring-boot.
Note that, as is recommended [1], the plugin’s ID does not include
“gradle”.

See gh-1567

[1] http://plugins.gradle.org/submit

spring-boot-samples/spring-boot-sample-web-ui/build.gradle

@@ -21,7 +21,7 @@ buildscript {
 apply plugin: 'java'
 apply plugin: 'eclipse'
 apply plugin: 'idea'
-apply plugin: 'spring-boot'
+apply plugin: 'org.springframework.boot.spring-boot'
 
 mainClassName = "sample.ui.SampleWebUiApplication"