Update .travis.yml

.travis.yml

@@ -1,23 +1,12 @@
+dist: bionic
 language: generic
 os: osx
 matrix:
   include:
     - os: linux
-      env: COMPILER_NAME=gcc CXX=g++-5 CC=gcc-5
-      addons:
-        apt:
-          packages:
-            - g++-5
-          sources: &sources
-            - llvm-toolchain-precise-3.8
-            - ubuntu-toolchain-r-test
+      env: COMPILER_NAME=gcc CXX=g++ CC=gcc
     - os: linux
       env: COMPILER_NAME=clang CXX=clang++-3.8 CC=clang-3.8
-      addons:
-        apt:
-          packages:
-            - clang-3.8
-          sources: *sources
 
 script: make all