Rework POM structure

Rework main build POM to be an aggregator pom that does not inherit
from any parent. Introduce new spring-boot-dependencies module to
act as a parent for both spring-boot-starter-parent and
spring-boot-parent.

spring-boot-cli/pom.xml

@@ -6,6 +6,7 @@
 		<groupId>org.springframework.boot</groupId>
 		<artifactId>spring-boot-parent</artifactId>
 		<version>0.5.0.BUILD-SNAPSHOT</version>
+		<relativePath>../spring-boot-parent</relativePath>
 	</parent>
 	<artifactId>spring-boot-cli</artifactId>
 	<packaging>jar</packaging>