Mention xargo in Readme.md and fix travis

.travis.yml

@@ -7,6 +7,7 @@ rust:
 
 script:
   - rustup component add rust-src
+  - cargo install xargo || echo "Skipping xargo install"
   - ./prepare_libcore.sh
   - ./build.sh
 

Readme.md

@@ -8,6 +8,7 @@
 $ git clone https://github.com/bjorn3/rustc_codegen_cranelift.git
 $ cd rustc_codegen_cranelift
 $ rustup override set nightly
+$ cargo install xargo
 $ git submodule update --init
 $ cargo build
 ```

build.sh

@@ -74,7 +74,7 @@ echo "[BUILD] core"
 time $RUSTC target/libcore/src/libcore/lib.rs --crate-type lib --crate-name core -Cincremental=target/incremental_core
 
 pushd xargo
-rm -r ~/.xargo/HOST
+rm -r ~/.xargo/HOST || true
 export XARGO_RUST_SRC=$(pwd)'/../target/libcore/src'
 time xargo build --color always
 rm -r target/