Fix rust nightly version on Travis CI

.travis.yml

@@ -5,17 +5,13 @@ rust:
 
 cache: 
   cargo: true
-  directories:
-    - $HOME/qemu
 
 before_script:
+  - rustup override set nightly-2018-04-01
   - rustup component add rust-src
   - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
   - (test -x $HOME/.cargo/bin/xargo || cargo install xargo)
   - cargo install-update -a
-  # setup qemu
-  - bash -e travis-qemu.sh
-  - export PATH=$PATH:$HOME/qemu/bin
 
 sudo: false
 
@@ -27,6 +23,7 @@ addons:
   apt:
     packages:
       - nasm
+      - qemu
 
 env:
   matrix: