travis: use newer ubuntu base

Scintilla needs C++ 17 which is supported since GCC 5. trusty/14.04 is ancient
and doesn't support C++17. Upgrade to bionic/18.04 which is old enough.

.travis.yml

@@ -1,7 +1,7 @@
 # we use both C and C++, so advertize C++
 language: cpp
 cache: ccache
-dist: trusty
+dist: bionic
 compiler:
   - gcc
 env:
@@ -14,7 +14,7 @@ env:
 before_install:
   - sudo apt-get update -qq
 install:
-  - sudo apt-get install -y intltool libtool
+  - sudo apt-get install -y intltool libtool autoconf-archive
   - test -n "$MINGW" || sudo apt-get install -y libgtk2.0-dev libgtk-3-dev
   - test -z "$MINGW" || sudo apt-get install -y mingw-w64-tools g++-mingw-w64-i686 gcc-mingw-w64-i686 binutils-mingw-w64-i686
   # fix broken pkg-config-crosswrapper, see https://bugs.launchpad.net/ubuntu/+source/mingw-w64/+bug/1327242