Switch to HTTPoison library

lib/opencpu/client.ex

lib/opencpu/delayed_calculation.ex

mix.exs

mix.lock

test/fixtures/vcr_cassettes/animation_flip_coin_getters.json

.../fixtures/vcr_cassettes/animation_flip_coin_graphics.json

test/fixtures/vcr_cassettes/bad_request.json

test/fixtures/vcr_cassettes/description.json

test/fixtures/vcr_cassettes/digest_hmac.json

test/fixtures/vcr_cassettes/flip_coin.json

test/fixtures/vcr_cassettes/github_package_not_found.json

test/fixtures/vcr_cassettes/prepare.json

test/fixtures/vcr_cassettes/response_with_na_values.json

test/opencpu/client_test.exs

test/opencpu/delayed_calculation_test.exs