feature flag upgrade code to spring 3.1.3

[nagkumar]

<dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-validation</artifactId>
    </dependency>
    <dependency>
        <groupId>javax.annotation</groupId>
        <artifactId>javax.annotation-api</artifactId>
        <version>1.3.2</version>
    </dependency>

these two extra depencides should be added 

https://github.com/thombergs/code-examples/tree/master/spring-boot/feature-flag