Update the Travis-CI test-build configuration.

Was: Ubuntu 16.04 ("Xenial") with LLVM 8.0
Now: Ubuntu 18.04 ("Bionic") with LLVM 9.0

Author: eeide

.travis.yml

@@ -10,16 +10,16 @@
 ###############################################################################
 
 sudo: required
-dist: xenial
+dist: bionic
 language: cpp
 
 before_install:
   - sudo scripts/travis_deps.sh
 
 script: >
-  CXX=/usr/lib/llvm-8/bin/clang++
-  CC=/usr/lib/llvm-8/bin/clang
-  ./configure --with-llvm=/usr/lib/llvm-8 &&
+  CXX=/usr/lib/llvm-9/bin/clang++
+  CC=/usr/lib/llvm-9/bin/clang
+  ./configure --with-llvm=/usr/lib/llvm-9 &&
   make -j2 &&
   make check
 

scripts/travis_deps.sh

@@ -16,7 +16,7 @@ set -eux
 
 apt-get update -qq
 
-# The Travis-CI Ubuntu 16.04 ("xenial") build environment has LLVM and Clang
+# The Travis-CI Ubuntu 18.04 ("bionic") build environment has LLVM and Clang
 # preinstalled.  They interfere with our installations of these packages.
 apt-get remove -y -qq \
     clang \
@@ -28,23 +28,23 @@ apt-get install -y -qq \
     software-properties-common \
     wget
 
-# Set up for installing LLVM 8.0.
+# Set up for installing LLVM 9.0.
 # See <https://wiki.ubuntu.com/ToolChain>.
 # See <http://llvm.org/apt/>.
 add-apt-repository -y \
     ppa:ubuntu-toolchain-r/test
 add-apt-repository -y \
-    'deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main'
+    'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
 wget -O - http://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
 apt-get update -qq
 
-# Install LLVM 8.0.  See file "INSTALL.md".
+# Install LLVM 9.0.  See file "INSTALL.md".
 apt-get install -y -qq \
-    llvm-8 \
-    llvm-8-dev \
-    clang-8 \
-    libclang-8-dev \
-    clang-format-8 \
+    llvm-9 \
+    llvm-9-dev \
+    clang-9 \
+    libclang-9-dev \
+    clang-format-9 \
     libedit-dev
 
 # Install other C-Reduce dependencies.  See file "INSTALL.md".