Upgrade baseline to Spring Boot 2.1 line(2.1.4)

The Spring Boot 2.0.x is finished basic maintenance. Hence; we will update baseline version to Spring Boot 2.1.4. (Support Spring Boot 2.2 line on Travis CI)

* https://spring.io/blog/2019/04/03/spring-boot-2-0-9-released

### Related Issue

* #317 