fix(namespace): revert xml namespace change

- The namespace was changed to http://camunda.org/bpmn in the last
release.
  This introduces a regression as changing the namespace breaks both the
  camunda modeler and the engine.

- For reasons of backwards compatibility, the namespace must not be
changed.

- Reverted to old namespace http://activity.org/bpmn

Related to CAM-1148
Merge branch 'master' of github.com:camunda/docs.camunda.org