mix deps.update ex_doc

Under Elixir 1.7.4 on my machine (which was just the version I had
installed for whatever reason), I couldn't run mix hex.publish because
I'd get the error

• (RuntimeError) module RemoteIp.Headers.Forwarded was not compiled with flag --docs (ex_doc) lib/ex_doc/retriever.ex:147: ExDoc.Retriever.export_docs?/1 (ex_doc) lib/ex_doc/retriever.ex:131: ExDoc.Retriever.get_module/2 (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2 (ex_doc) lib/ex_doc/retriever.ex:111: ExDoc.Retriever.docs_from_modules/2 (ex_doc) lib/ex_doc.ex:71: ExDoc.generate_docs/3 (ex_doc) lib/mix/tasks/docs.ex:117: Mix.Tasks.Docs.run/3 (mix) lib/mix/task.ex:316: Mix.Task.run_task/3 (hex) lib/mix/tasks/hex.publish.ex:170: Mix.Tasks.Hex.Publish.docs_task/2

Apparently this was being caused by an old version of ex_doc, per
https://github.com/elixir-lang/elixir/issues/8034. Updating ex_doc let
me publish v0.1.5 without any further issue.