Add application version info to JAR manifest.

According to [Spring Boot's reference guide](http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#boot-features-banner), a custom `banner.txt` should support the `${application.version}` and `${application.formatted-version}` placeholders. However, these only work if the application's manifest contains an `Implementation-Version` field. This patch configures the `maven-jar-plugin` in the `spring-boot-starter-parent` POM to provide the necessary field.