Fix multiple Stripe-Version headers when creating ephemeral key

[jayjun]

Creating an ephemeral key requires passing in a Stripe API version given by the iOS or Android SDK (see documentation). However, Stripe.EphemeralKey.create returns an error because multiple Stripe-Version headers are sent.

Stripe.EphemeralKey.create(%{customer: "cus_xxxxxxxxxxxxxx"}, "2019-05-16")

{:error,
 %Stripe.Error{
   code: :invalid_request_error,
   extra: %{
     http_status: 400,
     raw_error: %{
       "message" => "Invalid Stripe API version: 2019-05-16, 2019-12-03",
       "type" => "invalid_request_error"
     }
   },
   message: "Invalid Stripe API version: 2019-05-16, 2019-12-03",
   request_id: nil,
   source: :stripe,
   user_message: nil
 }}

This pull request overrides the default API version set using the one passed in, and updates links to the latest documentation.

Fixes #606.

[coveralls]

Coverage remained the same at 89.855% when pulling a5f0df2 on jayjun:version into 60d69dd on code-corps:master.

[snewcomer]

Great find!!