Generate ssso-merged-uberon.obo

[fbastian]

Many ontologies in the repository have been updated. I tried to generate the merged composite ontology myself, using the Makefile and owltools. Everything works fine, except that I miss the core UBERON life stages (e.g., class declaration for UBERON:0004730, etc).

Problems:
http://purl.obolibrary.org/obo/uberon/subsets/life-stages-minimal.obo is not accessible (error NoSuchKeyThe specified key does not exist.)
And anyway, I guess this life-stages-minimal.obo already contains the xrefs to species-specific life stage ontologies.

ping @cmungall: can you please tell me what am I missing to get the core UBERON classes declared in the merged life stage ontology?
(Note: see this PR for the updates: #72
But also I have a locally modified FBdv version that I would like to merge, so I'd like to do the merge myself)