Load google/api_client late to avoid dep.

    Updates the google/compute.rb module so that it requires the
    'google/api_client' dependency late. This avoids having to
    require (and install) proprietary google client API code
    to use Fog for other providers. (previously, even when using
    AWS I'd still have to install google-client-api)
    
    ---
    
    I understand code reuse is important but I think the fact that
    we recently added google-api-client as a top level dependency
    sets a bad precedent and perhaps highlights the need to move further
    towards "fog modules" for each provider to avoid these types of
    situations.