Ensure junebug logging service doesn't fail for invalid characters

[rudigiesler]

See this exception:

2017-04-22T17:07:34.197830599Z /usr/local/lib/python2.7/site-packages/twisted/python/log.py:136:err
2017-04-22T17:07:34.197850244Z /usr/local/lib/python2.7/site-packages/twisted/python/threadable.py:53:sync
2017-04-22T17:07:34.197863311Z /usr/local/lib/python2.7/site-packages/twisted/python/log.py:286:msg
2017-04-22T17:07:34.197875809Z /usr/local/lib/python2.7/site-packages/twisted/logger/_legacy.py:154:publishToNewObserver
2017-04-22T17:07:34.197888171Z --- <exception caught here> ---
2017-04-22T17:07:34.197900604Z /usr/local/lib/python2.7/site-packages/twisted/logger/_observer.py:131:__call__
2017-04-22T17:07:34.197912892Z /usr/local/lib/python2.7/site-packages/twisted/logger/_legacy.py:93:__call__
2017-04-22T17:07:34.197925351Z /usr/local/lib/python2.7/site-packages/junebug/logging_service.py:84:__call__
2017-04-22T17:07:34.197937620Z /usr/local/lib/python2.7/site-packages/twisted/python/log.py:86:callWithContext
2017-04-22T17:07:34.197949877Z /usr/local/lib/python2.7/site-packages/twisted/python/context.py:118:callWithContext
2017-04-22T17:07:34.197962300Z /usr/local/lib/python2.7/site-packages/twisted/python/context.py:81:callWithContext
2017-04-22T17:07:34.197974539Z /usr/local/lib/python2.7/site-packages/junebug/logging_service.py:77:_log_to_file
2017-04-22T17:07:34.197987120Z /usr/local/lib/python2.7/json/__init__.py:244:dumps
2017-04-22T17:07:34.197999058Z /usr/local/lib/python2.7/json/encoder.py:207:encode
2017-04-22T17:07:34.198011037Z /usr/local/lib/python2.7/json/encoder.py:270:iterencode
2017-04-22T17:07:34.198023085Z ]
2017-04-22T17:07:34.198035010Z Traceback (most recent call last):
2017-04-22T17:07:34.198047055Z   File "/usr/local/lib/python2.7/site-packages/twisted/logger/_observer.py", line 131, in __call__
2017-04-22T17:07:34.198059577Z     observer(event)
2017-04-22T17:07:34.198071368Z   File "/usr/local/lib/python2.7/site-packages/twisted/logger/_legacy.py", line 93, in __call__
2017-04-22T17:07:34.198083780Z     self.legacyObserver(event)
2017-04-22T17:07:34.198107529Z   File "/usr/local/lib/python2.7/site-packages/junebug/logging_service.py", line 84, in __call__
2017-04-22T17:07:34.198121308Z     log.callWithContext(self.log_context, self._log_to_file, event)
2017-04-22T17:07:34.198133328Z   File "/usr/local/lib/python2.7/site-packages/twisted/python/log.py", line 86, in callWithContext
2017-04-22T17:07:34.198145762Z     return context.call({ILogContext: newCtx}, func, *args, **kw)
2017-04-22T17:07:34.198157786Z   File "/usr/local/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
2017-04-22T17:07:34.198170446Z     return self.currentContext().callWithContext(ctx, func, *args, **kw)
2017-04-22T17:07:34.198182473Z   File "/usr/local/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
2017-04-22T17:07:34.198194759Z     return func(*args,**kw)
2017-04-22T17:07:34.198206461Z   File "/usr/local/lib/python2.7/site-packages/junebug/logging_service.py", line 77, in _log_to_file
2017-04-22T17:07:34.198218759Z     self.logfile.write(json.dumps(data) + '\n')
2017-04-22T17:07:34.198230977Z   File "/usr/local/lib/python2.7/json/__init__.py", line 244, in dumps
2017-04-22T17:07:34.198243080Z     return _default_encoder.encode(obj)
2017-04-22T17:07:34.198254992Z   File "/usr/local/lib/python2.7/json/encoder.py", line 207, in encode
2017-04-22T17:07:34.198267007Z     chunks = self.iterencode(o, _one_shot=True)
2017-04-22T17:07:34.198278859Z   File "/usr/local/lib/python2.7/json/encoder.py", line 270, in iterencode
2017-04-22T17:07:34.198290939Z     return _iterencode(o, 0)
2017-04-22T17:07:34.198304390Z UnicodeDecodeError: 'utf8' codec can't decode byte 0xf1 in position 146: invalid continuation byte```