Upgrade to Native Build Tools Plugin 0.9.13

[wilkinsona]

As part of this, we should straighten out the dependency management. It's currently split across spring-boot-dependencies and spring-boot-parent with the same version declared in those two places. I think we have two options:

1. Declare it all in spring-boot-dependencies
2. Declare the version in gradle.properties

The problem with 1 is that the version of native-gradle-plugin declared in spring-boot-dependencies will have no effect on the version used in an application's Gradle-based build as Gradle doesn't support externalised plugin management. For this reason, I think 2 is the better choice.