Unlock oauth2

[ueberauth]

Unlock oauth2

[sobolevn]

Tried to change it to ~> 0.8, now tests do not pass.

Generated ueberauth_vk app
.

1. test default callback phase (Ueberauth.Strategy.VKTest)
   test/ueberauth/strategy/vk_test.exs:58
   ** (ArgumentError) argument error
   stacktrace:
   :erlang.iolist_to_binary(#Reference<0.0.2.771>)
   lib/poison/parser.ex:35: Poison.Parser.parse/2
   lib/poison/parser.ex:51: Poison.Parser.parse!/2
   lib/poison.ex:83: Poison.decode!/2
   (oauth2) lib/oauth2/response.ex:32: OAuth2.Response.new/3
   (oauth2) lib/oauth2/request.ex:23: OAuth2.Request.request/6
   (oauth2) lib/oauth2/client.ex:216: OAuth2.Client.get_token/4
   (oauth2) lib/oauth2/client.ex:231: OAuth2.Client.get_token!/4
   (ueberauth_vk) lib/ueberauth/strategy/vk.ex:47: Ueberauth.Strategy.VK.handle_callback!/1
   (ueberauth) lib/ueberauth/strategy.ex:299: Ueberauth.Strategy.run_callback/2
   test/test_helper.exs:1: SpecRouter.plug_builder_call/2
   test/ueberauth/strategy/vk_test.exs:65: (test)

..

Finished in 0.3 seconds
4 tests, 1 failure