use Ubuntu 18 for travis

.travis.yml

 language: cpp
-dist: trusty
+dist: bionic
 git:
   submodules: false
 addons:
   ssh_known_hosts:
   - github.com
   apt:
-    sources:
-      - ubuntu-toolchain-r-test
+   # sources:
+   #   - ubuntu-toolchain-r-test
     packages:
-      - gcc-6
-      - g++-6
+    #  - gcc-6
+    #  - g++-6
       - libevent-dev
       - libsqlite3-dev
 before_install:
 - git submodule update --init --recursive
-- sudo ln -s /usr/bin/gcc-6 /usr/local/bin/gcc
-- sudo ln -s /usr/bin/g++-6 /usr/local/bin/g++
-- g++ --version
+#- sudo ln -s /usr/bin/gcc-6 /usr/local/bin/gcc
+#- sudo ln -s /usr/bin/g++-6 /usr/local/bin/g++
+#- g++ --version
 - wget -O - https://github.com/premake/premake-core/releases/download/v5.0.0-alpha12/premake-5.0.0-alpha12-linux.tar.gz | tar zfx -
 - wget -O - https://www.lua.org/ftp/lua-5.3.5.tar.gz | tar zfx -; cd lua-5.3.5; sudo make linux install; cd ..
 script: