Add dependency entry for the configuration processor

[tokuhirom]

When I'm writing an application on spring-boot, I'm writing my configuration class, annotated by @ConfigurationProperties.

Enabling spring-boot-configuration-processor is bothersome.

I recommend enabling spring-boot-configuration-processor by default.

http://docs.spring.io/spring-boot/docs/1.4.1.RELEASE/reference/html/configuration-metadata.html#configuration-metadata-annotation-processor

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-configuration-processor</artifactId>
    <optional>true</optional>
</dependency>

[snicoll]

Need #128

[snicoll]

@tokuhirom this was already declined in #196 but adding an option at least is a great idea.