travis: should test building qemu-arm as well.

.travis.yml

@@ -7,12 +7,13 @@ before_script:
   - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
   - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
   - sudo apt-get update -qq
-  - sudo apt-get install -y python3.3 python3 gcc-4.7 gcc-arm-none-eabi
+  - sudo apt-get install -y python3.3 python3 gcc-4.7 gcc-arm-none-eabi qemu-system
 
 script:
   - make -C unix CC=gcc-4.7
   - make -C unix-cpy CC=gcc-4.7
   - make -C bare-arm
+  - make -C qemu-arm
   - make -C stmhal
   - (cd tests && MICROPY_CPYTHON3=python3.3 ./run-tests)