Update rustc-serialize to 0.3.25

[hcl-dr]

Fixes lifetime issue in rustc-serialize 0.3.24

Compiling rustc-serialize v0.3.24
error[E0310]: the parameter type `T` may not live long enough
    --> rustc-serialize-0.3.24/src/serialize.rs:1155:5
     |
1155 |     fn decode<D: Decoder>(d: &mut D) -> Result<Cow<'static, T>, D::Error> {
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |     |
     |     the parameter type `T` must be valid for the static lifetime...
     |     ...so that the type `T` will meet its required lifetime bounds...
     |
note: ...that is required by this bound
    --> rustlib/src/rust/library/alloc/src/borrow.rs:180:30
     |
180  | pub enum Cow<'a, B: ?Sized + 'a>
     |                              ^^
help: consider adding an explicit lifetime bound
     |
1151 | impl<'a, T: ?Sized + 'static> Decodable for Cow<'a, T>
     |                    +++++++++