software/plantuml: update to 1.2020.15.war
Rendering of some diagrams to png is slightly different, images are not same size
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![software/plantuml/test/data/test_class_diagram.png](https://lab.nexedi.com/lpgeneau/slapos/-/raw/bd062c1d29e38c20d0b0999f2013cc8b4228d841/software/plantuml/test/data/test_class_diagram.png)
Rendering of some diagrams to png is slightly different, images are not same size
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