Fix error when compiling rusttype

[caulagi]

I got the following error with compiling rusttype when I did vagrant up after a fresh checkout

    default: error[E0277]: the trait bound `std::sync::Arc<[u8]>: std::convert::From<std::boxed::Box<[u8]>>` is not satisfied
    default:    --> /home/cratesfyi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusttype-0.4.0/src/lib.rs:146:34
    default:     |
    default: 146 |         SharedBytes::ByArc(bytes.into())
    default:     |                                  ^^^^ the trait `std::convert::From<std::boxed::Box<[u8]>>` is not implemented for `std::sync::Arc<[u8]>`
    default:     |
    default:     = help: the following implementations were found:
    default:               <std::sync::Arc<T> as std::convert::From<T>>
    default:     = note: required because of the requirements on the impl of `std::convert::Into<std::sync::Arc<[u8]>>` for `std::boxed::Box<[u8]>`
    default: error[E0277]: the trait bound `std::sync::Arc<[u8]>: std::convert::From<std::vec::Vec<u8>>` is not satisfied
    default:    --> /home/cratesfyi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusttype-0.4.0/src/lib.rs:151:34
    default:     |
    default: 151 |         SharedBytes::ByArc(bytes.into())
    default:     |                                  ^^^^ the trait `std::convert::From<std::vec::Vec<u8>>` is not implemented for `std::sync::Arc<[u8]>`
    default:     |
    default:     = help: the following implementations were found:
    default:               <std::sync::Arc<T> as std::convert::From<T>>
    default:     = note: required because of the requirements on the impl of `std::convert::Into<std::sync::Arc<[u8]>>` for `std::vec::Vec<u8>`
    default: error
    default: : aborting due to 2 previous errors
    default: error: Could not compile `rusttype`.
    default: Build failed, waiting for other jobs to finish...

This seems very similar to https://github.com/redox-os/rusttype/issues/71. Updating the nightly version used by vagrant fixed the issue for me. I see that the latest travis build on master uses rustc 1.24.1 (d3ae9a9e0 2018-02-27), which could explain why build didn't fail.

[onur]

@caulagi thanks! I forgot to update Vagrantfile.