👷 removing Clang 3.9 and 4.0

These versions are still not working properly: https://travis-ci.org/nlohmann/json/jobs/251675281

Author: nlohmann

.travis.yml

@@ -183,17 +183,17 @@ matrix:
 
   - os: linux
     compiler: gcc
-    env:
-      - COMPILER=g++-6
-      - CXXFLAGS=-std=c++1z
+    env: COMPILER=g++-7
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
-        packages: g++-6
+        packages: g++-7
 
   - os: linux
     compiler: gcc
-    env: COMPILER=g++-7
+    env:
+      - COMPILER=g++-7
+      - CXXFLAGS=-std=c++17
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
@@ -229,22 +229,6 @@ 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: