Updates to be able to create a dmg on MacOSX
Created by: thibauthourlier
For these changes to work, it needs changes to be merged from https://github.com/thibauthourlier/ensembl-otter branch feature/mac_dmg into https://github.com/Ensembl/ensembl-otter branch master It also needs a PR to be accepted for Zircon. This change is not required but could solve some problems