plantuml.jar upgraded from 7931 to 7958

plantuml.jar upgraded from 7931 to 7958

Old (7931) version cause errors on UMLs like https://gist.github.com/vistrcm/8565f0dc0ec55c7264a0

You can see result for old version here:
![plantuml_7931](https://f.cloud.github.com/assets/211080/194037/84a5c220-7f99-11e2-9727-76f450bebdd3.png)

PlantUML 7958 fix it:
![plantuml_7958](https://f.cloud.github.com/assets/211080/194042/9d52262e-7f99-11e2-909f-e8a80dd2c4cf.png)