feat: ExMachina.start/2: return a supervisor from Application callback

[aeruder]

Description

ExMachina.start/2: return a supervisor from Application callback

We're actually returning the PID for an Agent rather than a
supervisor. In addition to being against the erlang callback spec:

    The function is to return {ok,Pid} or {ok,Pid,State}, where Pid
    is the pid of the top supervisor and State is any term. If
    omitted, State defaults to []. If the application is stopped
    later, State is passed to Module:prep_stop/1.

Some tools like phoenix live dashboard will send the
application supervisor supervisor-y messages and end up crashing the
root application tree.

    ** (FunctionClauseError) no function clause matching in Agent.Server.handle_call/3
        (elixir 1.14.5) lib/agent/server.ex:11: Agent.Server.handle_call(:which_children, {#PID<0.22073.22>, #Reference<0.3953567925.3593732098.137638>}, %{})
        (stdlib 3.17) gen_server.erl:721: :gen_server.try_handle_call/4
        (stdlib 3.17) gen_server.erl:750: :gen_server.handle_msg/6
        (stdlib 3.17) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
    Last message (from #PID<0.22073.22>): :which_children

This change makes ExMachina return a supervisor with a single child -
the ExMachina.Sequence. Have also moved to use Agent to get the
child_spec/1 for free in ExMachina.Sequence.

This has been tested per the tool versions file (1.7.4 + otp 21).

 lib/ex_machina.ex          | 7 ++++++-
 lib/ex_machina/sequence.ex | 4 +++-
 2 files changed, 9 insertions(+), 2 deletions(-)

Before

Afterwards

Tests

test@742e43ef2efc:/ex_machina$ mix test
[...]
==> ex_machina
Compiling 16 files (.ex)
Generated ex_machina app
The database for ExMachina.TestRepo has been dropped
The database for ExMachina.TestRepo has been created
....................................................................................

Finished in 0.2 seconds
84 tests, 0 failures

Randomized with seed 422393
test@742e43ef2efc:/ex_machina$ elixir --version
Erlang/OTP 21 [erts-10.3.5.19] [source] [64-bit] [smp:5:5] [ds:5:5:10] [async-threads:1]

Elixir 1.7.4 (compiled with Erlang/OTP 21)

[doomspork]

Thank you @aeruder! Now that ExMachina has been transferred to the BEAM Community we'll work on gettin these PRs incorporated and new releases flowing. Thanks again for the contribution!