upgrade to crossbar version compatible with Python 3.6

[ghost]

Currently, simpl-modelservice requires crossbar==17.10.1 and Python 3.6.

Crossbar < 18.0 requires the external enum34 package that ghosts the enum library available in core for python 3.6+. (the package does not implement the complete API available in 3.6+)

If someone tries to use the 3.6+ enum core library and they could spend a couple weeks trying to figure out why their code is crashing.

[frankwiles]

If I remember correctly this isn't an easy upgrade, but we should look into it.