Determine HTTPoison module at compile time

[jtormey]

Follow up from #37.

Moving this check to a module attribute means it will be executed at compile time, which removes the need for adding mix to :extra_applications in mix.exs.

All tests pass, and the module is determined correctly based on env:

elixir-auth-google $ mix run -e "IO.inspect ElixirAuthGoogle.inject_poison()"
HTTPoison
elixir-auth-google $ MIX_ENV=test mix run -e "IO.inspect ElixirAuthGoogle.inject_poison()"
ElixirAuthGoogle.HTTPoisonMock

[nelsonic]

Hi @jtormey, the code you've added looks like it should work. 🤔
Need to investigate why the build doesn't pass when back at my desk ... https://travis-ci.com/github/dwyl/elixir-auth-google/builds/227901294#L289

[nelsonic]

Checked out @jtormey's branch and the tests pass on localhost: ✅

I think we could merge this PR and confirm that it will pass on main. 💭

[nelsonic]

Build passes on main:https://travis-ci.com/github/dwyl/elixir-auth-google/builds/228131972#L337

[nelsonic]

Thanks again @jtormey 🚀