Travis: Update build env to Ubuntu Focal

1/ Update to Focal environment
2/ Make build stop on first error
3/ Remove unnecessary PPAs

.travis.yml

@@ -1,12 +1,10 @@
 language: c
 
-dist:  bionic
+dist:  focal
 
 addons:
   apt:
     sources:
-      - sourceline: 'ppa:dosemu2/ppa'
-      - sourceline: 'ppa:stsp-0/djgpp'
       - sourceline: 'ppa:tkchia/build-ia16'
     packages:
       - gcc-ia16-elf

travis_build.sh

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 if [ -z "${TRAVIS_BUILD_DIR}" ] ; then
   TRAVIS_BUILD_DIR=$(pwd)
 fi