upgrade to crossbar version compatible with Python 3.7

[ghost]

Crossbario will need to be upgraded to a Python 3.7 compatible version in order to run simpl-modelservice under Python 3.7.

Traceback (most recent call last):
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/bin/crossbar", line 6, in <module>
    from crossbar.controller.cli import run
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/lib/python3.7/site-packages/crossbar/controller/cli.py", line 62, in <module>
    from crossbar.controller.node import _read_release_pubkey, _read_node_pubkey
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/lib/python3.7/site-packages/crossbar/controller/node.py", line 64, in <module>
    from crossbar.worker.router import RouterRealm
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/lib/python3.7/site-packages/crossbar/worker/router.py", line 66, in <module>
    from crossbar.worker.testee import WebSocketTesteeServerFactory, \
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/lib/python3.7/site-packages/crossbar/worker/testee.py", line 45, in <module>
    from crossbar.worker.worker import NativeWorkerSession
  File "/Users/janee/.virtualenvs/simpl-calc-model-3.7/lib/python3.7/site-packages/crossbar/worker/worker.py", line 191
    def start_profiler(self, profiler=u'vmprof', runtime=10, async=True, details=None):
                                                                 ^
SyntaxError: invalid syntax

Also see #10.