Build for Redox on Travis

.cargo/config

+[target.x86_64-unknown-redox]
+linker = "x86_64-unknown-redox-gcc"

.gitignore

 .*
+!.cargo
+!.travis
 cachegrind.*
 perf.*
 target

.travis.yml

 language: rust
 rust:
   - nightly
-sudo: false
+sudo: true
 notifications:
   email: false
+env:
+  - REDOX=
+  - REDOX=1 CC=x86_64-unknown-redox-gcc CARGO_ARGS='--target=x86_64-unknown-redox'
+before_install:
+  - if [ $REDOX ]; then ./.travis/redox-toolchain.sh; fi
 script:
-  - cargo build
-  - cargo test
-  - bash examples/run_examples.sh
+  - cargo build $CARGO_ARGS 
+  - if [ ! $REDOX ]; then cargo test; fi
+  - if [ ! $REDOX ]; then bash examples/run_examples.sh; fi

.travis/redox-toolchain.sh

+#!/bin/bash
+
+rustup target add x86_64-unknown-redox
+sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys AA12E97F0881517F
+sudo add-apt-repository 'deb https://static.redox-os.org/toolchain/apt /'
+sudo apt-get update -qq
+sudo apt-get install -y x86-64-unknown-redox-gcc