* src/main/java/com/tikal/jenkins/plugins/multijob/MultiJobBuilder.java: Fix MatrixBuild support for SubBuild

Build was used in multiple place instead of AbstractBuild which means it failed when the sub build was a MatrixBuild.
Symptom was that the MatrixBuild was started but never joined and did not showed up as active in the MultiJob page.

Signed-off-by: Nicolas Morey-Chaisemartin <nmorey@kalray.eu>
Merge pull request #42 from nmorey/matrix-build-support

* src/main/java/com/tikal/jenkins/plugins/multijob/MultiJobBuilder.java:...
checked...