Adding `cbc-mac` `v0.2.0-rc.4` as a dependency doesn't compile

[dcormier]

Adding cbc-mac v0.2.0-rc.4 as a dependency doesn't compile. I added the dependency to an otherwise empty lib crate and the result is this:

> cargo clippy         
    Checking cbc-mac v0.2.0-rc.4
error[E0432]: unresolved import `digest::common::BlockSizes`
  --> /Users/danielc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cbc-mac-0.2.0-rc.4/src/block_api.rs:10:14
   |
10 |     common::{BlockSizes, InnerInit, InnerUser},
   |              ^^^^^^^^^^
   |              |
   |              no `BlockSizes` in the root
   |              help: a similar name exists in the module: `BlockSizeUser`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `cbc-mac` (lib) due to 1 previous error

[tarcieri]

Will be fixed by #258

[tarcieri]

@dcormier please try cbc-mac v0.2.0-rc.5