core: Make antrun plugin version explicit

Currently we don't explicitly specify the version of the maven ant
plugin that we use. This means that we use an old version. This patch
udpates to version 1.7 and makes it explicit in the parent POM.

Change-Id: Iede68182eaa101813abccb3d534fe297651ff648
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>