[Travis] Add testing with -std=c++20

Also, bump default compiler to Clang 7, which supports C++20 to some
extent.

.travis.yml

@@ -93,11 +93,15 @@ matrix:
     ##########################################################################
     # Build with variations in the configuration
     ##########################################################################
-    # With C++17 instead of C++14, on Clang
+    # With C++17, on Clang
     - env: UNIT_TESTS=true COMPILER=default BOOST_VERSION=default CMAKE_OPTIONS="-DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_STANDARD_REQUIRED=ON"
       addons: *defaults
 
-    # With C++17 instead of C++14, on GCC
+    # With C++20, on Clang
+    - env: UNIT_TESTS=true COMPILER=default BOOST_VERSION=default CMAKE_OPTIONS="-DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON"
+      addons: *defaults
+
+    # With C++17, on GCC
     - env: UNIT_TESTS=true COMPILER=g++-6     BOOST_VERSION=default CMAKE_OPTIONS="-DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_STANDARD_REQUIRED=ON"
       addons: { apt: { packages: ["g++-6", "valgrind"], sources: ["ubuntu-toolchain-r-test"] } }
 
@@ -173,7 +177,7 @@ install:
   ############################################################################
   # Setup default versions and override CXX set by Travis if needed
   ############################################################################
-  - if [[ "${COMPILER}" == "default" ]]; then COMPILER=clang++-5.0; fi
+  - if [[ "${COMPILER}" == "default" ]]; then COMPILER=clang++-7.0; fi
   - if [[ "${BOOST_VERSION}" == "default" ]]; then BOOST_VERSION=1.66.0; fi
 
   - if [[ "${COMPILER}" != "" ]]; then export CXX=${COMPILER}; fi
@@ -234,7 +238,9 @@ install:
       elif [[ "${CXX}" == "clang++-3.8" ]]; then LLVM_VERSION="3.8.1";
       elif [[ "${CXX}" == "clang++-3.9" ]]; then LLVM_VERSION="3.9.1";
       elif [[ "${CXX}" == "clang++-4.0" ]]; then LLVM_VERSION="4.0.1";
-      elif [[ "${CXX}" == "clang++-5.0" ]]; then LLVM_VERSION="5.0.1";
+      elif [[ "${CXX}" == "clang++-5.0" ]]; then LLVM_VERSION="5.0.2";
+      elif [[ "${CXX}" == "clang++-6.0" ]]; then LLVM_VERSION="6.0.1";
+      elif [[ "${CXX}" == "clang++-7.0" ]]; then LLVM_VERSION="7.0.1";
       fi
 
       LLVM_URL="http://llvm.org/releases/${LLVM_VERSION}/llvm-${LLVM_VERSION}.src.tar.xz"