Upgrade to clang-6 and gcc-8.

The new compilers are undocumented but available.

.travis.yml

@@ -43,14 +43,15 @@ matrix:
     addons:
       apt:
         sources:
-          - llvm-toolchain-trusty-5.0
+          - llvm-toolchain-trusty-6.0
+          - ubuntu-toolchain-r-test
         packages:
           - exiv2
           - libexiv2-dev
-          - clang-5.0
+          - clang-6.0
     compiler: clang
     env:
-      - CONFIG=clang5
+      - CONFIG=clang6
   - os: linux
     addons:
       apt:
@@ -59,19 +60,19 @@ matrix:
         packages:
           - exiv2
           - libexiv2-dev
-          - g++-7
+          - g++-8
     compiler: gcc
     env:
-      - CONFIG=gcc7
+      - CONFIG=gcc8
 
 # Only build the command-line tool for now
 script:
   - if [ "$CONFIG" = "normal" ] ; then CFLAGS='-Wall -Wextra -Werror -O3 -DENABLE_NLS=1'; fi
   - if [ "$CONFIG" = "no_nls" ] ; then CFLAGS='-Wall -Wextra -O3'; fi
   - if [ "$CONFIG" = "sanitize" ] ; then CFLAGS='-g -O0 -fsanitize=address -fsanitize=undefined -DENABLE_NLS=1'; LDFLAGS='$(CFLAGS)'; fi
   - if [ "$CONFIG" = "coverage" ] ; then CFLAGS='-g -O0 --coverage'; LDFLAGS='$(CFLAGS)'; fi
-  - if [ "$CONFIG" = "clang5" ] ; then CFLAGS='-Wall -Wextra -Werror -O3 -DENABLE_NLS=1'; CC=clang-5.0; CXX=clang++-5.0; fi
-  - if [ "$CONFIG" = "gcc7" ] ; then CFLAGS='-Wall -Wextra -Werror -O3 -DENABLE_NLS=1'; CC=gcc-7; CXX=g++-7; fi
+  - if [ "$CONFIG" = "clang6" ] ; then CFLAGS='-Wall -Wextra -Werror -O3 -DENABLE_NLS=1'; CC=clang-6.0; CXX=clang++-6.0; fi
+  - if [ "$CONFIG" = "gcc8" ] ; then CFLAGS='-Wall -Wextra -Werror -O3 -DENABLE_NLS=1'; CC=gcc-8; CXX=g++-8; fi
 
     # Explicitly point to brew's gettext installation on OS X
   - if [ "$TRAVIS_OS_NAME" = "osx" ] ; then CFLAGS="$CFLAGS -I/usr/local/opt/gettext/include"; LDFLAGS="$LDFLAGS -L/usr/local/opt/gettext/lib -lintl"; fi