update travis

.travis.yml

@@ -2,17 +2,14 @@ dist: trusty
 sudo: required
 language: python
 
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - g++-6
-
 python:
     - "3.5"
     - "3.6"
+before_install:
+  - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+  - sudo apt-get update -qq
 install:
+  - sudo apt-get install -qq gcc-6 g++-6
   - pip install -r requirements.txt
   - pip install pyquickhelper --no-deps
 before_script: