Revert support for phoenix 1.6 from the master branch

[chewnoill]

• Reverts support for phoenix 1.6
  Torch won't support phoenix 1.6 officially until 4.0

[cpjolicoeur]

@chewnoill see my note here regarding Torch 3/4 and branching: #286 (comment)

[cpjolicoeur]

@chewnoill as part of this PR can we update the README to include copy/notes that Torch 3/mainline only supports Phx 1.5 and below, and then also point people to the v4 branch for Phx 1.6+ support.

[chewnoill]

elixir 1.10 builds are failing now, Not sure what changed here:

Generating docs...
** (ExDoc.Retriever.Error) module Mix.Tasks.Torch.Gen.Html is not defined/available
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:89: ExDoc.Retriever.docs_chunk/1
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:55: ExDoc.Retriever.get_module/2
(elixir 1.10.4) lib/enum.ex:3343: Enum.flat_map_list/2
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:42: ExDoc.Retriever.docs_from_modules/2
(ex_doc 0.28.0) lib/ex_doc.ex:24: ExDoc.generate_docs/3
(ex_doc 0.28.0) lib/mix/tasks/docs.ex:359: anonymous fn/7 in Mix.Tasks.Docs.run/3
(elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
(ex_doc 0.28.0) lib/mix/tasks/docs.ex:358: Mix.Tasks.Docs.run/3

[cpjolicoeur]

elixir 1.10 builds are failing now, Not sure what changed here:

Generating docs...
** (ExDoc.Retriever.Error) module Mix.Tasks.Torch.Gen.Html is not defined/available
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:89: ExDoc.Retriever.docs_chunk/1
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:55: ExDoc.Retriever.get_module/2
(elixir 1.10.4) lib/enum.ex:3343: Enum.flat_map_list/2
(ex_doc 0.28.0) lib/ex_doc/retriever.ex:42: ExDoc.Retriever.docs_from_modules/2
(ex_doc 0.28.0) lib/ex_doc.ex:24: ExDoc.generate_docs/3
(ex_doc 0.28.0) lib/mix/tasks/docs.ex:359: anonymous fn/7 in Mix.Tasks.Docs.run/3
(elixir 1.10.4) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
(ex_doc 0.28.0) lib/mix/tasks/docs.ex:358: Mix.Tasks.Docs.run/3

I merged this earlier: #285

I didn't notice in the changelog that it ways 0.28.0 requires Elixir 1.10+.

I'll revert that PR and then you can rebase.

[cpjolicoeur]

Some typos here. "Torch" and "Phoenix"

[cpjolicoeur]

ok @chewnoill #288 merged. you can rebase this now