stop using our browserchannel fork, switch back to upstream

* Our fork had a single change to ensure \u2028 and \u2029 characters
  were escaped before being snt over the wire
* Our changes were accepted upstream 2 years ago [1] and [2] so there's
  no need to maintain the fork

[1] https://github.com/josephg/node-browserchannel/commit/a7b1f37510c026acfa8e704e48319f2c0ad8c511
[2] https://github.com/josephg/node-browserchannel/commit/d9d4dc56f63af02b72bfcd30c7c5069f5a1fdee3
Merge pull request #3 from conversation/switch-to-upstream-browserchannel

stop using our browserchannel fork, switch back to upstream