Make CircleCI install GCC ARM 4.9 instead of 5.x or 6.x

Author: bridadan

circle.yml

@@ -15,6 +15,6 @@ dependencies:
         - cd ../testproject && git init
         - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
         - sudo apt-get -y update
-        - sudo apt-get -y install  gcc-arm-embedded
+        - sudo apt-get -y install gcc-arm-embedded=4.9-2015-q3-update
     override:
         - cd ../testproject/mbed-os && pip install -r requirements.txt