Bugfix/enable usage of runtime.exs to configure

[walu-lila]

What is the Purpose?

• Enables usage of runtime.exs to configure the package
• Update to use the Config module instead of Mix.Config to remove deprecated warnings
• Upgrade the dependency versions

What was the approach?

Elixir added support for using a runtime.exs file to configure the application. This enables
configuration to be resolved at application startup. However, the API Key was being stored as
a module attribute (@key) which was being resolved at compile time. This caused the following error:
{:error, %{message: "The supplied authentication is invalid", status: 401}}
This was due to the API Key not being set to a value during compilation. To fix this the module attribute
was removed and the API Key is resolved when needed.

Upgraded the dependency version to fix the following error

Because "the lock" specifies tesla 1.9.0 and every version of at_ex depends on tesla ~> 1.2.1, the lock is incompatible with at_ex.
And because your app depends on the lock, no version of at_ex is allowed.
So, because your app depends on at_ex ~> 0.20.22, version solving failed.

Are there any concerns to addressed further before or after merging this PR?

Run mix deps.get

Mentions?

@manuelgeek
@sigu
@kamalogudah
@TraceyOnim

Issue(s) affected?

Configuration using runtime.exs does not work #95

[lenileiro]

[okothkongo]

@walu-lila let us 🚀 it

[manuelgeek]

@okothkongo could you kindly merge handle updates on hex?
I added you as lib owner