Avoid warnings after split of Kernel.Typespec into {Kernel,Code}.Typespec

[milmazz]

The idea behind this PR is avoid this kind of warnings under Elixir 1.7.0-dev:

λ mix test
Compiling 3 files (.ex)
Excluding tags: [no_formatter: true]

..warning: Kernel.Typespec.beam_specs/1 is deprecated, please use Code.Typespec.fetch_specs/1 instead
  (elixir) lib/kernel/typespec.ex:48: Kernel.Typespec.beam_specs/1
  (ex_doc) lib/ex_doc/retriever.ex:481: ExDoc.Retriever.get_specs/1
  (ex_doc) lib/ex_doc/retriever.ex:195: ExDoc.Retriever.get_module_info/2
  (ex_doc) lib/ex_doc/retriever.ex:166: ExDoc.Retriever.generate_node/3
  (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2
  (ex_doc) lib/ex_doc/retriever.ex:116: ExDoc.Retriever.docs_from_modules/2
  test/ex_doc/formatter/epub/templates_test.exs:30: ExDoc.Formatter.EPUB.TemplatesTest.get_module_page/1
  test/ex_doc/formatter/epub/templates_test.exs:66: ExDoc.Formatter.EPUB.TemplatesTest."test module_page outputs summaries"/1
  (ex_unit) lib/ex_unit/runner.ex:312: ExUnit.Runner.exec_test/1
  (stdlib) timer.erl:166: :timer.tc/1
  (ex_unit) lib/ex_unit/runner.ex:251: anonymous fn/4 in ExUnit.Runner.spawn_test/3

[michalmuskala]

I think I would prefer using if Code.ensure_loaded?(Code.Typespec) do as the condition instead.

[michalmuskala]

Thank you! ❤️

[michalmuskala]

@milmazz I'm sorry for merging it and not waiting for you. It was mostly out of the habit and that big green button is so appealing 😅

[milmazz]

@michalmuskala Don't worry at all. Thanks for the feedback btw.