Allow api_version to be set from config

[stoyle]

The docs states you should be able to set api_version through config, as far as I can tell this should be valid:

config :stripity_stripe,
  api_version: "2020-08-27",
  api_key: "sk_test_xxx"

However api_version is not read. With this change it is.

[coveralls]

Coverage increased (+0.01%) to 90.256% when pulling d8d2b0e on stoyle:api_version_from_config into b0d1651 on code-corps:master.

[snewcomer]

This is a great addition!