Update rustc-serialize to 0.1.5

This is in response to rust-lang/rust@16f01cc - the change was addressed
in rust-lang/rustc-serialize@1561cca, versioned 0.1.5.

Update dependency to this version.
auto merge of #1089 : bheart/cargo/master, r=alexcrichton

Builds break on recent versions of rustc.

This is in response to rust-lang/rust@16f01cc - the change was addressed
in rust-lang/rustc-serialize@1561cca, versioned 0.1.5.

Update dependency accordingly.