Upgrade Travis CI conf

.travis.yml

@@ -9,7 +9,12 @@ branches:
   only:
     - master
     - stable
-before_script:
-    - sudo add-apt-repository -y ppa:boost-latest/ppa
-    - sudo apt-get update -qq
-    - sudo apt-get install libboost-thread1.55-dev libboost-system1.55-dev
+sudo: false
+cache:
+    - apt
+addons:
+  apt:
+    sources:
+    - boost-latest
+    packages:
+    - libboost-thread1.55-dev libboost-system1.55-dev