[*] update travis config

.travis.yml

 sudo: required
-language: cpp
+language: c
 compiler: gcc
-dist: trusty
+dist: xenial
 notifications:
   email: false
 
 script:
-    - export CC=gcc-5
     - mkdir build
     - cd build 
     - cmake ../
@@ -16,5 +15,3 @@ addons:
   apt:
     sources:
     - ubuntu-toolchain-r-test
-    packages:
-    - gcc-5