Use GCC 5.4 for travis builds (#518)

.travis.yml

@@ -30,10 +30,10 @@ language: cpp
 compiler: clang
 
 before_install:
-  - curl --retry 10 --retry-max-time 120 -L "https://launchpad.net/gcc-arm-embedded/4.9/4.9-2015-q2-update/+download/gcc-arm-none-eabi-4_9-2015q2-20150609-linux.tar.bz2" | tar xfj -
+  - curl --retry 10 --retry-max-time 120 -L "https://launchpad.net/gcc-arm-embedded/5.0/5-2016-q2-update/+download/gcc-arm-none-eabi-5_4-2016q2-20160622-linux.tar.bz2" | tar xfj -
 
 install:
-  - export PATH=$PATH:$PWD/gcc-arm-none-eabi-4_9-2015q2/bin
+  - export PATH=$PATH:$PWD/gcc-arm-none-eabi-5_4-2016q2/bin
 
 before_script: arm-none-eabi-gcc --version
 script: ./.travis.sh