Revert "Updated gem spec to require json rather than multi_json"

This reverts commit 66638b25d76a5eb737c9a41ed9b4847841549d50.

Since google-api-client was added as a dependency in ca945a0432a1fae4b1,
multi_json is a de facto dependency of fog, so this is a needless layer.
If #1034 is still an issue, I'd be happy to ship a version of multi_json
that requires rubygems >= 1.3.5.