ci: use ubuntu 18.04 for builds

Author: JaCzekanski

.travis.yml

@@ -7,14 +7,17 @@ matrix:
   include:
   - name: Linux Clang6
     os: linux
+    dist: bionic
     services: docker
     script: "./.travis/linux/run-container.sh 6"
   - name: Linux Clang8
     os: linux
+    dist: bionic
     services: docker
     script: "./.travis/linux/run-container.sh 8"
   - name: Android
     os: linux
+    dist: bionic
     services: docker
     before_install: openssl aes-256-cbc -K $encrypted_7333c7dd5b15_key -iv $encrypted_7333c7dd5b15_iv -in android/avocado.keystore.enc -out android/avocado.keystore -d
     script: "./.travis/android/run-container.sh"

.travis/linux/build.sh

@@ -7,8 +7,8 @@ export OS=linux64
 export DATE=`date +%Y%m%d`
 export COMMIT=`git rev-parse --short=7 HEAD`
 export ARTIFACT=avocado-$OS-$DATE-$COMMIT
-export CC="ccache clang-6.0 -fcolor-diagnostics"
-export CXX="ccache clang++-6.0 -fcolor-diagnostics"
+export CC="ccache cc -fcolor-diagnostics"
+export CXX="ccache c++ -fcolor-diagnostics"
 
 # Configure cache
 ccache --set-config=sloppiness=pch_defines,time_macros