Update .travis.yml to use Ubuntu Bionic, remove redundant packages

.travis.yml

@@ -1,8 +1,8 @@
-sudo: required
-dist: trusty
+os: linux
+dist: bionic
+arch: amd64
 
 language: cpp
-os: linux
 compiler: gcc
   
 branches:
@@ -12,19 +12,11 @@ branches:
 
 addons:
   apt:
-    sources:
-      - ubuntu-toolchain-r-test
     packages:
-      - gcc-5
-      - g++-5
       - zlib1g-dev
       - build-essential
       - libmysqlclient-dev
       - liblua5.1-0-dev
-      - cmake
-
-before_install:
-  - export CXX="g++-5" CC="gcc-5"
   
 before_script:
   - mkdir build