Travis-CI: Move from Ubuntu 14.04 to 18.04

Ubuntu 14.04 reached EOL for mainstream support in April 2019, and I'm
about to add a dependency on a version of SDL newer than the one in
14.04. Instead of going via 16.04, which only has about a year of
support lifetime left itself, let's go directly to 18.04, currently the
latest LTS release.

Update build-dependencies accordingly.

Signed-off-by: Simon McVittie <[email protected]>

.travis.yml

@@ -1,6 +1,6 @@
 language: cpp
 sudo: required
-dist: trusty
+dist: bionic
 
 addons:
   homebrew:

scripts/travis/install.sh

@@ -20,7 +20,7 @@ case "${host}" in
 			libsdl2-dev \
 			libjpeg-turbo8-dev \
 			zlib1g-dev \
-			libpng12-dev
+			libpng-dev
 		;;
 
 	(i686-w64-mingw32)
@@ -41,15 +41,12 @@ case "${host}" in
 			libjpeg-turbo8-dev:i386 \
 			zlib1g-dev:i386 \
 			libc6-dev:i386 \
-			libpng12-dev:i386 \
-			g++-multilib \
-			g++-4.8-multilib \
-			gcc-4.8-multilib \
+			libpng-dev:i386 \
+			cpp \
 			g++ \
-			g++-4.8 \
+			g++-multilib \
 			gcc \
-			gcc-4.8 \
-			cpp \
-			cpp-4.8
+			gcc-multilib \
+			${NULL+}
 		;;
 esac