MSRV build on CI is broken due to dependencies

[eldruin]

With Rust 1.31.1 the builds fail due to:

Compiling scopeguard v1.1.0
     Running `rustc --crate-name scopeguard /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/scopeguard-1.1.0/src/lib.rs --color never --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=2c583fe15a6f6ee9 -C extra-filename=-2c583fe15a6f6ee9 --out-dir /home/travis/build/rust-embedded/rust-sysfs-gpio/target/debug/deps -L dependency=/home/travis/build/rust-embedded/rust-sysfs-gpio/target/debug/deps --cap-lints allow`
error[E0658]: use of unstable library feature 'try_from' (see issue #33417)
  --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/byteorder-1.4.2/src/lib.rs:74:5
   |
74 |     convert::TryInto, fmt::Debug, hash::Hash, ptr::copy_nonoverlapping, slice,
   |     ^^^^^^^^^^^^^^^^
   Compiling slab v0.4.2
     Running `rustc --crate-name slab /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/slab-0.4.2/src/lib.rs --color never --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=aeeeff3fc23ce2a0 -C extra-filename=-aeeeff3fc23ce2a0 --out-dir /home/travis/build/rust-embedded/rust-sysfs-gpio/target/debug/deps -L dependency=/home/travis/build/rust-embedded/rust-sysfs-gpio/target/debug/deps --cap-lints allow`

error[E0658]: use of unstable library feature 'int_to_from_bytes' (see issue #52963)
    --> /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/byteorder-1.4.2/src/lib.rs:1959:9
     |
1959 |         u16::from_be_bytes(buf[..2].try_into().unwrap())
     |         ^^^^^^^^^^^^^^^^^^