Update ex_doc to 0.20.2 to get the versions dropdown on hexdocs

[michallepicki]

I had to bump elixir requirement to 1.7 because of this error when generating docs:

** (ExDoc.Retriever.Error) ExDoc 0.19+ requires Elixir v1.7 and later. For earlier Elixir versions, make sure to depend on {:ex_doc, "~> 0.18.0"}
    (ex_doc) lib/ex_doc/retriever.ex:84: ExDoc.Retriever.docs_chunk/1
    (ex_doc) lib/ex_doc/retriever.ex:63: ExDoc.Retriever.get_module/2
    (elixir) lib/enum.ex:1003: Enum.flat_map_list/2
    (ex_doc) lib/ex_doc/retriever.ex:43: ExDoc.Retriever.docs_from_modules/2
    (ex_doc) lib/ex_doc.ex:30: ExDoc.generate_docs/3
    (ex_doc) lib/mix/tasks/docs.ex:298: anonymous fn/6 in Mix.Tasks.Docs.run/3
    (elixir) lib/enum.ex:1826: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ex_doc) lib/mix/tasks/docs.ex:297: Mix.Tasks.Docs.run/3

[michallepicki]

Rebased on current master