Travis: fix gcc arm version to 6

Use download from official website, to have
fixed version, rather than latest.

The official support for current mbed OS is
GCC ARM 6.

.travis.yml

@@ -18,6 +18,7 @@ cache:
   pip: true
   directories:
     - $HOME/.cache/apt
+    - $HOME/gcc-arm-none-eabi-6-2017-q2-update
 
 before_install:
   - bash -c "$STATUS" pending "Local $NAME testing is in progress"
@@ -160,7 +161,11 @@ matrix:
       env: NAME=mbed2-NXP
       install:
         # Install dependencies
-        - sudo apt-get install gcc-arm-embedded
+        - export GCC_DIR=$HOME/gcc-arm-none-eabi-6-2017-q2-update
+        - export GCC_ARCHIVE=$HOME/gcc-arm-none-eabi-6-2017-q2-update-linux.tar.bz2
+        - export GCC_URL=https://developer.arm.com/-/media/Files/downloads/gnu-rm/6-2017q2/gcc-arm-none-eabi-6-2017-q2-update-linux.tar.bz2?revision=2cc92fb5-3e0e-402d-9197-bdfc8224d8a5?product=GNU%20Arm%20Embedded%20Toolchain,64-bit,,Linux,6-2017-q2-update
+        - if [ ! -e $GCC_DIR/bin/arm-none-eabi-gcc ]; then wget $GCC_URL -O $GCC_ARCHIVE; tar xfj $GCC_ARCHIVE -C $HOME; fi
+        - export PATH=$PATH:$GCC_DIR/bin
         - pip install -r requirements.txt
         # Print versions we use
         - arm-none-eabi-gcc --version