Add the jar build plugin version.

[Salmela]

resolves #40

[jamo]

FYI: For some reason I would have used maven-shade-plugin, e.g: https://github.com/testmycode/tmc-core/blob/master/pom.xml#L75-L87

[mikkomaa]

LGTM.

[coveralls]

Coverage remained the same at 0.0% when pulling bba5ee3 on build-plugin-aleksi into 7deb23f on master.

[coveralls]

Coverage remained the same at 0.0% when pulling bba5ee3 on build-plugin-aleksi into 7deb23f on master.

[coveralls]

Coverage remained the same at 0.0% when pulling f4f4425 on build-plugin-aleksi into 7deb23f on master.