Update gcc-arm-embedded PPA in Travis

* ppa:terry.guo/gcc-arm-embedded has been deprecated in favor of ppa:team-gcc-arm-embedded/ppa
* this also means switching from 4.9.3.2015q3 to 6-2017q2

Author: amq

.travis.yml

@@ -28,7 +28,7 @@ script:
   - python tools/project.py -S
   - python tools/build_travis.py
 before_install:
-  - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
+  - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
   - sudo add-apt-repository -y ppa:libreoffice/libreoffice-4-2
   - sudo apt-get update -qq
   - sudo apt-get install -qq gcc-arm-none-eabi doxygen --force-yes