phoenix docs error

gistfile1.txt

jhancock@air:~/phoenix$ MIX_ENV=docs mix run release_docs.exs
==> ex_doc
warning: the dependency ex_doc requires Elixir "~> 0.15.0" but you are running on v1.0.0
Compiled lib/ex_doc/cli.ex
Compiled lib/ex_doc/markdown.ex
Compiled lib/ex_doc.ex
Compiled lib/ex_doc/markdown/earmark.ex
Compiled lib/ex_doc/markdown/hoedown.ex
Compiled lib/ex_doc/markdown/pandoc.ex

== Compilation error on file lib/mix/tasks/docs.ex ==
** (CompileError) lib/mix/tasks/docs.ex:66: function 'nil?'/1 undefined
    (stdlib) lists.erl:1336: :lists.foreach/2
    (stdlib) erl_eval.erl:657: :erl_eval.do_apply/6
    (elixir) src/elixir.erl:175: :elixir.erl_eval/3
    (elixir) src/elixir.erl:163: :elixir.eval_forms/4
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/3
    (elixir) src/elixir.erl:175: :elixir.erl_eval/3

could not compile dependency ex_doc, mix compile failed. You can recompile this dependency with `mix deps.compile ex_doc` or update it with `mix deps.update ex_doc`
jhancock@air:~/phoenix$