Fix rust 1.32 build

[haplo]

https://travis-ci.org/github/haplo/rust-bch/jobs/697559764

socket2 fails to compile:

   Compiling socket2 v0.3.12

error[E0658]: use of unstable library feature 'maybe_uninit' (see issue #53491)

 --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/socket2-0.3.12/src/sockaddr.rs:2:22

  |

2 | use std::mem::{self, MaybeUninit};

  |                      ^^^^^^^^^^^

error[E0658]: use of unstable library feature 'maybe_uninit' (see issue #53491)

  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/socket2-0.3.12/src/sockaddr.rs:46:27

   |

46 |         let mut storage = MaybeUninit::<sockaddr_storage>::uninit();

   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0599]: no function or associated item named `uninit` found for type `std::mem::MaybeUninit<libc::unix::linux_like::sockaddr_storage>` in the current scope

  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/socket2-0.3.12/src/sockaddr.rs:46:27

   |

46 |         let mut storage = MaybeUninit::<sockaddr_storage>::uninit();

   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function or associated item not found in `std::mem::MaybeUninit<libc::unix::linux_like::sockaddr_storage>`

error: aborting due to 3 previous errors

Some errors occurred: E0599, E0658.

For more information about an error, try `rustc --explain E0599`.

error: Could not compile `socket2`.

Once it works enable the build in ,travis.yml.

[haplo]

ring is a dependency that only supports stable rust, so rust-bch is not going to be able to support older Rust versions unless the dependency is dropped.