Metrics server broken with prometheus-client 0.8

Currently this is fixed by limiting the version to <0.8 in https://github.com/chime-experiment/coco/pull/197/commits/1f5db51cdb80d5a572b2d3dff260a0d828517e7b

```
0.91s$ PYTHONPATH=. pytest -xs --cov-report term-missing:skip-covered --cov=coco tests/test_*.py

============================= test session starts ==============================

platform linux -- Python 3.7.1, pytest-4.3.1, py-1.7.0, pluggy-0.8.0

rootdir: /home/travis/build/chime-experiment/coco, inifile:

plugins: cov-2.9.0

collecting ...

==================================== ERRORS ====================================

___________________ ERROR collecting tests/test_blacklist.py ___________________

ImportError while importing test module '/home/travis/build/chime-experiment/coco/tests/test_blacklist.py'.

Hint: make sure your test modules/packages have valid Python names.

Traceback:

tests/test_blacklist.py:6: in

from coco.blacklist import Blacklist

coco/__init__.py:19: in

from .request_forwarder import RequestForwarder, ExternalForward, CocoForward

coco/request_forwarder.py:13: in

from .metric import start_metrics_server

coco/metric.py:11: in

from prometheus_client.exposition import (

E ImportError: cannot import name '_ThreadingSimpleServer' from 'prometheus_client.exposition' (/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/prometheus_client/exposition.py)

=========================== 1 error in 0.33 seconds ============================

The command "PYTHONPATH=. pytest -xs --cov-report term-missing:skip-covered --cov=coco tests/test_*.py" exited with 1.
cache.2

store build cache
```