Fix multiversion build

Only generate for heads of branches, not for every tag

Change-Id: I2e0a379cf725c264f675db151eadbfedbae41f76
diff --git a/Makefile b/Makefile
index 2accb01..7dd066d 100644
--- a/Makefile
+++ b/Makefile
@@ -58,7 +58,7 @@
 	rm -rf $(VENV_NAME)
 
 # build multiple versions
-multiversion: $(VENV_NAME) Makefile | prep $(OTHER_REPO_DOCS)
+multiversion: $(VENV_NAME) Makefile
 	source $</bin/activate ; set -u ;\
   sphinx-multiversion "$(SOURCEDIR)" "$(BUILDDIR)/multiversion" $(SPHINXOPTS)
 	cp "$(SOURCEDIR)/_templates/meta_refresh.html" "$(BUILDDIR)/multiversion/index.html"