Start the :ssl application

Adds the :ssl application to the list of applications in mix.exs.

When using the default httpc adapter, attempting to GET an https URL would result in the error ** (Tesla.Error) adapter error: :econnrefused.

The :econnrefused error is actually hiding the root cause, which is :ssl_not_started.

It appears that the httpc adapter is not automatically starting the :ssl application.