Fix failure of ARM-AAPCS travis job

Up till now we have been installing arm-gcc using apt from the team-gcc-arm-embedded ppa.

As discussed in #1453 and implemented in #1504, we have decided to lock travis to use the specific version of the toolchain that is documented in the READMEs of relevant platforms. However, the PPA no longer hosts this version, apt fails to install arm-gcc and as a result the job fails too.

This commit changes the travis job to wget an installation tarball for the desired version, instead of trying to install via apt.

.travis.yml

@@ -40,13 +40,13 @@ before_script:
       arm-none-eabi-gcc --version ;
     fi
 
-  ## Install mainline ARM toolchain.  gcc-arm-none-eabi is available
-  ## in Ubuntu >= 14.04, but this external PPA is needed for 12.04.
-  ## Install srecord
+  ## Install mainline ARM toolchain and srecord.
   - if [ ${BUILD_ARCH:-0} = arm-aapcs ] ; then
-      sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa &&
-      sudo apt-get -qq update &&
-      sudo apt-get -qq install gcc-arm-embedded=5-2015q4-1~precise1 srecord &&
+      sudo apt-get -qq install srecord &&
+      $WGET https://launchpad.net/gcc-arm-embedded/5.0/5-2015-q4-major/+download/gcc-arm-none-eabi-5_2-2015q4-20151219-linux.tar.bz2 &&
+      tar xjf gcc-arm-none-eabi-5_2-2015q4-20151219-linux.tar.bz2 -C /tmp/ &&
+      sudo cp -f -r /tmp/gcc-arm-none-eabi-5_2-2015q4/* /usr/local/ &&
+      rm -rf /tmp/gcc-arm-none-eabi-* gcc-arm-none-eabi-*-linux.tar.bz2 &&
       arm-none-eabi-gcc --version ;
     fi