Test 64-bit linux

.travis.yml

@@ -28,7 +28,7 @@ matrix:
     - env: TARGET=thumbv7m-linux-eabi
     - env: TARGET=x86_64-apple-darwin
       os: osx
-env: TARGET=x86_64-unknown-linux-gnu
+    - env: TARGET=x86_64-unknown-linux-gnu
 
 before_install:
   - test "$TRAVIS_OS_NAME" = "osx" || docker run --rm --privileged multiarch/qemu-user-static:register