Fix logicgraph generation
Fix logicgraph generation

The previous code worked flawlessly on a mac, but my ubuntu system
could not execute `system` properly. `system` returned `nil` with `$?`
being nil as well. With the backtick notation it works.

ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub