update travis script

.travis.yml

@@ -1,5 +1,7 @@
 sudo: false
 
+dist: xenial
+
 language: rust
 
 rust: nightly-2019-01-01
@@ -12,11 +14,9 @@ cache:
   cargo: true
   directories:
     - $HOME/Library/Caches/Homebrew
+    - $TRAVIS_BUILD_DIR/qemu-3.1.0
 
 addons:
-  apt:
-    packages:
-      - qemu
   homebrew:
     packages:
       - qemu
@@ -48,7 +48,10 @@ install:
       brew install aarch64-none-elf;
     fi;
     fi
-
+  - if [ $TRAVIS_OS_NAME = linux ]; then
+      wget https://download.qemu.org/qemu-3.1.0.tar.xz && tar xvJf qemu-3.1.0.tar.xz > /dev/null && cd qemu-3.1.0 && ./configure --target-list=$ARCH-softmmu && make && cd ..;
+      export PATH=$PATH:$PWD/qemu-3.1.0/$ARCH-softmmu;
+    fi
 
 before_script:
   - rustup component add rust-src