Calling #flush will not flush the cache if using v1 of the API

[pfeiffer]

Related to #17.

After the changes in #21, it seems that #flush is no longer flushing the cache, if the client is using the version 1 of the API.

client = Storyblok::Client.new(api_version: 1, ..)

client.flush # Will *not* flush cache

I think it that the caching and flushing should either be compatible with both v1 and v2 of the API or support should be dropped for specifying the API version if the behavior of the public interface of the gem cannot be kept the same between the two API versions.