Update version to 2.1.0-SNAPSHOT on master branch

[kazuki43zoo]

The Spring Boot 2.0.x is finished basic maintenance. Hence; we create 2.0.x branch and update version to 2.1.0-SNAPSHOT on master branch. The mybatis-spring-boot 2.1 line will support Spring Boot 2.1 and 2.2+(does not release GA version yet).

• https://spring.io/blog/2019/04/03/spring-boot-2-0-9-released

Related Issue

• Upgrade baseline to Spring Boot 2.1 line(2.1.4) #318