Update .travis.yml

.travis.yml

@@ -1,15 +1,19 @@
 sudo: required
 language: c++
   
+before_install:
+  - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
+  - sudo apt-get update -qq
+  - sudo apt-get install -qq gcc-arm-embedded --force-yes
+  
 addons:
     apt:
         sources:
-            - team-gcc-arm-embedded/ppa
+            
         packages:
             - lib32bz2-1.0
             - lib32ncurses5
             - lib32z1
-            - gcc-arm-embedded
             
 install:
     - sudo pip install -r mbed-os/requirements.txt