using external PlantUML server can break the testsuite

[robtaylor]

We don't have any control over changes to the external plantuml server used, so any changes deployed by the plantuml developers can break our test suite, making it fragile. (i.e. http://www.plantuml.com/plantuml see [core/publishers/html.py:43(https://github.com/doorstop-dev/doorstop/blob/develop/doorstop/core/publishers/html.py#L43))

Solution is for the test suite to install a version locked plantuml and use this.