bump generic-array dependents' versions

[FauxFaux]

Please update postgres-protocol to use newer versions of generic_arary. This is just a Cargo.toml change; no code changes are necessary.

Without this change, cargo build on my project either hangs indefinitely during resolution, or fails with odd errors about typenum, which I have been otherwise unable to resolve:

error: failed to select a version for `typenum` (required by `generic-array`):
all possible versions conflict with previously selected versions of `typenum`
  version 1.9.0 in use by typenum v1.9.0
  possible versions to select: 1.5.2, 1.5.1, 1.5.0

[sfackler]

Thanks!