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)

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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.
Add a custom log warning on load error.
Merge pull request #1797 from dprince/google_api_client

Load google/api_client late to avoid dep.