stack/erp5: add PyYAML in eggs

This makes yaml module to zope processes. The package was already
installed because it is an indirect dependency, but not in the working
set of zope processes

stack/erp5/buildout.cfg

@@ -540,6 +540,7 @@ eggs = ${neoppod:eggs}
   ${scipy:egg}
   ${scikit-learn:egg}
   ${scikit-image:egg}
+  ${python-PyYAML:egg}
   sympy
   more-itertools
   ${h5py:egg}