Allow Gradle properties to override bom properties

This commit allows the Gradle project’s properties to override the
properties in a Maven bom. For example if the bom contains the
following:

<properties>
	<spring.version>4.0.6.RELEASE</spring.version>
</properties>

<dependencyManagement>
	<dependencies>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-core</artifactId>
			<version>${spring.version}</version>
		</dependency>
	<dependencies>
</dependencyManagement>

Then setting the Gradle project’s spring.version property will override
the version of spring-core that’s provided.