No longer needed because it has been merged into the core.

Track a change to proxy handling in Autobahn / twisted for #477. There's probably a more elegant way, but this works for now... Lacking a test, but have verified it locally.

We don't need the HTTPProxyConfig object at all anymore, just a `dict` will do.

Have seen this error a couple of times during live operation:

```Shell
Unhandled error in Deferred:
[CRITICAL] (2018-05-08 13:43:42,705) in twisted[154] - Unhandled error in Deferred:

[CRITICAL] (2018-05-08 13:43:42,705) in twisted[154] -
Traceback (most recent call last):
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/venv/lib/python2.7/site-packages/twisted/internet/defer.py", line 150, in maybeDeferred
    result = f(*args, **kw)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/node/consumer.py", line 134, in convert_next_segment
    end=self.last_segment_end + self._segment_length
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/node/consumer.py", line 122, in get_segment
    sequence_number=self._segment_counter
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/documents/ebutt3.py", line 1004, in extract_segment
    document_segments=document_segments
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/documents/ebutt3_splicer.py", line 29, in __init__
    self._do_splice()
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/documents/ebutt3_splicer.py", line 66, in _do_splice
    merged_body = merged_body.merge(current_tt.body, self._dataset)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/bindings/__init__.py", line 974, in merge
    self._merge_deconflict_ids(element=other_elem, dest=merged_body, ids=ids)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/bindings/__init__.py", line 949, in _merge_deconflict_ids
    cls._merge_deconflict_ids(item.value, copied_elem, ids)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/bindings/__init__.py", line 949, in _merge_deconflict_ids
    cls._merge_deconflict_ids(item.value, copied_elem, ids)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/bindings/__init__.py", line 949, in _merge_deconflict_ids
    cls._merge_deconflict_ids(item.value, copied_elem, ids)
  File "/Users/megitn02/Code/ebu/ebu-tt-live-toolkit/ebu_tt_live/bindings/__init__.py", line 942, in _merge_deconflict_ids
    log.debug('processing child: {} of {}'.format(item.value, element))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa3' in position 1: ordinal not in range(128)
```

So commenting out that line. No idea how that character can appear in the relevant line, it's a £ sign.

Simplest fix for #481

This strips out unencodable characters and if not fixes, at least masks #483 by using `codecs.open` and telling it to ignore errors.