.travis.yml: Install gcc-arm-none-eabi with --force-yes for now.

There appears to be issue signature problem with the PPA package we use,
so workaround it this way for now. Warning: with broken signature, there's
always a possibility that PPA was hacked and ships trojaned binaries.

.travis.yml

@@ -12,7 +12,8 @@ before_script:
   - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
   - sudo dpkg --add-architecture i386
   - sudo apt-get update -qq || true
-  - sudo apt-get install -y python3 gcc-multilib gcc-arm-none-eabi pkg-config libffi-dev libffi-dev:i386 qemu-system mingw32
+  - sudo apt-get install -y python3 gcc-multilib pkg-config libffi-dev libffi-dev:i386 qemu-system mingw32
+  - sudo apt-get install -y --force-yes gcc-arm-none-eabi
   # For teensy build
   - sudo apt-get install realpath
   # For coverage testing