Plugin doesn't work with upcoming Elixir v1.10

[starbelly]

Originally reported by liveforeverx in Supersonido/rebar_mix#15

Environment

Elixir & Erlang/OTP versions (elixir --version): Elixir 1.10.0-dev (46f6ba5) (compiled with Erlang/OTP 22)
Operating system: Mac OS

Current behavior

Creating new application and using rebar_mix plugin for example to include jason library, like this in rebar.config:

{erl_opts, [debug_info]}.
{deps, [
{jason, "1.1.2"}
]}.

{plugins, [rebar_mix]}.
{provider_hooks, [
{pre, [{compile, {mix, find_elixir_libs}}]},
{post, [{compile, {mix, consolidate_protocols}}]}
]}.

{shell, [
{apps, [mylib]}
]}.

Expected behavior

Jason compiled.

Jason compiles with Elixir v1.9, but with upcoming Elixir v1.10 it produces an error:

rebar3 shell
...
Generated decimal app
===> Failed to read required .app file for processing the application 'decimal': no such file or directory

Tracing back it is dependent on this change:

• [Mix.Project] Make sure MIX_BUILD_PATH specifies only the build_path prefix and that env+target are still concatenated

So probably this change could fix this:

https://github.com/Supersonido/rebar_mix/blob/master/src/rebar_mix_builder.erl#L14-L15

to:

{env, [{"MIX_BUILD_PATH", filename:join(AppDir, "../../../")},
{"MIX_ENV", "default"}]}]) of

Or getting MIX_ENV from profiles (I see multiple profiles, but I don't know, which one to use).