Remove a dependency to be compatible with the new sphinx version

doc/sphinx/source/conf.py

@@ -39,7 +39,7 @@ import orgasm.version
 extensions = ['sphinx.ext.autodoc', 'sphinx.ext.todo', 
               'sphinx.ext.coverage', 'sphinx.ext.viewcode',
               'sphinx.ext.graphviz', 'sphinx.ext.inheritance_diagram',
-              'sphinx.ext.pngmath',
+#              'sphinx.ext.pngmath',
 #              'matplotlib.sphinxext.mathmpl',
 #              'matplotlib.sphinxext.only_directives',
 #              'matplotlib.sphinxext.plot_directive',