switch all Linux/clang jobs to Clang 6.0

Travis have removed llvm-toolchain-precise-3.7 from their list of
whitelisted sources for apt with
travis-ci/apt-source-safelist@e9d632e

Author: Daniel Kroening

.travis.yml

@@ -146,7 +146,7 @@ jobs:
         - EXTRA_CXXFLAGS="-DDEBUG"
       script: echo "Not running any tests for a debug build."
 
-    # Ubuntu Linux with glibc using clang++-3.7, no-debug mode
+    # Ubuntu Linux with glibc using clang++, no-debug mode
     - stage: Test different OS/CXX/Flags
       os: linux
       sudo: false
@@ -156,10 +156,10 @@ jobs:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.7
+            - llvm-toolchain-trusty-6.0
           packages:
             - libwww-perl
-            - clang-3.7
+            - clang-6.0
             - g++-5
             - libstdc++-5-dev
             - libubsan0
@@ -169,13 +169,13 @@ jobs:
         - ln -s /usr/bin/gcc-5 bin/gcc
         - ln -s /usr/bin/c++-5 bin/g++
         - export CCACHE_CPP2=yes
-      # env: COMPILER=clang++-3.7 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
+      # env: COMPILER=clang++ SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
       env:
-        - COMPILER="ccache /usr/bin/clang++-3.7"
+        - COMPILER="ccache /usr/bin/clang++-6.0"
         - EXTRA_CXXFLAGS="-Qunused-arguments -fcolor-diagnostics -DNDEBUG"
         - CCACHE_CPP2=yes
 
-    # Ubuntu Linux with glibc using clang++-3.7, debug mode, disable USE_DSTRING
+    # Ubuntu Linux with glibc using clang++, debug mode, disable USE_DSTRING
     - stage: Test different OS/CXX/Flags
       os: linux
       sudo: false
@@ -185,10 +185,10 @@ jobs:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise-3.7
+            - llvm-toolchain-trusty-6.0
           packages:
             - libwww-perl
-            - clang-3.7
+            - clang-6.0
             - g++-5
             - libstdc++-5-dev
             - libubsan0
@@ -197,9 +197,9 @@ jobs:
         - ln -s /usr/bin/gcc-5 bin/gcc
         - ln -s /usr/bin/g++-5 bin/g++
         - export CCACHE_CPP2=yes
-      # env: COMPILER=clang++-3.7 SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
+      # env: COMPILER=clang++ SAN_FLAGS="-fsanitize=undefined -fno-sanitize-recover=undefined,integer -fno-omit-frame-pointer"
       env:
-        - COMPILER="ccache /usr/bin/clang++-3.7"
+        - COMPILER="ccache /usr/bin/clang++-6.0"
         - EXTRA_CXXFLAGS="-Qunused-arguments -fcolor-diagnostics -DDEBUG -DUSE_STD_STRING"
         - CCACHE_CPP2=yes
       script: echo "Not running any tests for a debug build."