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 afa9b025e9a1c47488,
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.