Allow failures on nightly rust

This was added because for an unknown reason the OS X nightly built is not
able to detect the Rust's package manager

Usage:
    cargo <command> [<args>...]
    cargo [options]

Options:
    -h, --help          Display this message
    -V, --version       Print version info and exit
    --list              List installed commands
    --explain CODE      Run `rustc --explain CODE`
    -v, --verbose ...   Use verbose output
    -q, --quiet         No output printed to stdout
    --color WHEN        Coloring: auto, always, never
    --frozen            Require Cargo.lock and cache are up to date
    --locked            Require Cargo.lock is up to date

Some common cargo commands are (see all commands with --list):
    build       Compile the current project
    clean       Remove the target directory
    doc         Build this project's and its dependencies' documentation
    new         Create a new cargo project
    init        Create a new cargo project in an existing directory
    run         Build and execute src/main.rs
    test        Run the tests
    bench       Run the benchmarks
    update      Update dependencies listed in Cargo.lock
    search      Search registry for crates
    publish     Package and upload this project to the registry
    install     Install a Rust binary

See 'cargo help <command>' for more information on a specific command. binary after installing rust

.travis.yml

@@ -73,3 +73,7 @@ notifications:
   email:
     on_success: change
     on_failure: change
+
+matrix:
+  allow_failures:
+    - rust: nightly