👷 adding two recent Clang versions

Maybe the simple approach is working now - at least it is in https://github.com/taocpp/PEGTL/blob/master/.travis.yml

.travis.yml

@@ -229,6 +229,22 @@ matrix:
     env: LLVM_VERSION=3.8.1
     compiler: clang
 
+  - os: linux
+    compiler: clang
+    env: COMPILER=clang++-3.9
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-3.9']
+        packages: clang-3.9
+
+  - os: linux
+    compiler: clang
+    env: COMPILER=clang++-4.0
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-4.0']
+        packages: clang-4.0
+
 #  - os: linux
 #    addons:
 #      apt:

README.md

@@ -709,7 +709,7 @@ json j_from_msgpack = json::from_msgpack(v_msgpack);
 
 Though it's 2016 already, the support for C++11 is still a bit sparse. Currently, the following compilers are known to work:
 
-- GCC 4.9 - 6.0 (and possibly later)
+- GCC 4.9 - 7.1 (and possibly later)
 - Clang 3.4 - 3.9 (and possibly later)
 - Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0 (and possibly later)
 - Microsoft Visual C++ 2017 / Build Tools 15.1.548.43366 (and possibly later)
@@ -738,6 +738,7 @@ The following compilers are currently used in continuous integration at [Travis]
 | GCC 4.9.4       | Ubuntu 14.04.5 LTS           | g++-4.9 (Ubuntu 4.9.4-2ubuntu1~14.04.1) 4.9.4 |
 | GCC 5.4.1       | Ubuntu 14.04.5 LTS           | g++-5 (Ubuntu 5.4.1-2ubuntu1~14.04) 5.4.1 20160904 |
 | GCC 6.3.0       | Ubuntu 14.04.5 LTS           | g++-6 (Ubuntu/Linaro 6.3.0-18ubuntu2~14.04) 6.3.0 20170519 |
+| GCC 7.1.0       | Ubuntu 14.04.5 LTS           | g++-7 (Ubuntu 7.1.0-5ubuntu2~14.04) 7.1.0
 | Clang 3.6.0     | Ubuntu 14.04.5 LTS           | clang version 3.6.0 (tags/RELEASE_360/final) |
 | Clang 3.6.1     | Ubuntu 14.04.5 LTS           | clang version 3.6.1 (tags/RELEASE_361/final) |
 | Clang 3.6.2     | Ubuntu 14.04.5 LTS           | clang version 3.6.2 (tags/RELEASE_362/final) |