ruby: Fix SHA256 hashes of several versions.

According to @zimbatm, he got the SHA256 hashes via nix-prefetch-git.

However, fetchFromGitHub doesn't use Git to fetch the sources but uses
fetchzip under the hood, so we get plain source directories in the Nix
store, which in turn are hashed.

Tested by:

nix-build --no-out-link -E 'map (x:
    (builtins.getAttr x (import ./.  {})).src
) [ "ruby_1_9_3" "ruby_2_0_0" "ruby_2_1_0" "ruby_2_1_1" "ruby_2_1_2"
    "ruby_2_1_3" "ruby_2_1_6" "ruby_2_1_7" "ruby_2_2_0" "ruby_2_2_2"
    "ruby_2_2_3"
]'

Signed-off-by: aszlig <aszlig@redmoonstudios.org>