Cargo.lock: bump dependencies (#397)

Updates the following dependencies:

    $ cargo update                                                                                                                                      101 ↵
    Updating crates.io index
     Locking 9 packages to latest compatible versions
    Updating autocfg v1.4.0 -> v1.5.0
    Updating cfg-if v1.0.0 -> v1.0.1
    Updating ecdsa v0.17.0-rc.4 -> v0.17.0-rc.5
    Updating libc v0.2.172 -> v0.2.174
    Updating r-efi v5.2.0 -> v5.3.0
    Updating signature v3.0.0-rc.1 -> v3.0.0-rc.2
    Updating syn v2.0.101 -> v2.0.104
    Updating zerocopy v0.8.25 -> v0.8.26
    Updating zerocopy-derive v0.8.25 -> v0.8.26

Author: tarcieri