main, travis: add qemu to run tests in

This makes sure we'll catch bugs that occur only on ARM hardware.

.travis.yml

@@ -8,7 +8,7 @@ before_install:
   - echo "deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-7 main"            | sudo tee -a /etc/apt/sources.list
   - echo "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu trusty main" | sudo tee -a /etc/apt/sources.list
   - sudo apt-get update -qq
-  - sudo apt-get install llvm-7-dev clang-7 gcc-arm-none-eabi --allow-unauthenticated -y
+  - sudo apt-get install llvm-7-dev clang-7 gcc-arm-none-eabi qemu-system-arm --allow-unauthenticated -y
   - sudo ln -s /usr/bin/clang-7 /usr/local/bin/cc # work around missing -no-pie in old GCC version
 
 install: