Yank pre-0.3.20 versions

[KamilaBorowska]

They don't compile due to patterns in functions without bodies.

error[E0642]: patterns aren't allowed in functions without bodies
   --> /home/xfix/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-serialize-0.3.19/src/serialize.rs:147:45
    |
147 | ...                   &f_name: &str,
    |                       ^^^^^^^ pattern not allowed in function without body

This affects cargo update -Z minimal-versions.