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

This reverts commits: 66638b25d76a5eb737c9a41ed9b4847841549d50, 3f0314dbd1972051e0f705e8f83813e7e2305e46, and 18ce4b7eca41ea75d45564466e4d44076128981e.

Since `google-api-client` was added as a dependency in ca945a0432a1fae4b1cc8bd650a09eefd0082303, `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`.