updated README

.travis.yml

@@ -46,6 +46,14 @@ matrix:
           branch_pattern: coverity_scan
       env: COMPILER=g++-5
 
+    - os: linux
+      compiler: gcc
+      addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test']
+          packages: ['g++-6', 'valgrind']
+      env: COMPILER=g++-6
+
     # from https://github.com/travis-ci/travis-ci/issues/6120
     - os: linux
       env:
@@ -61,14 +69,6 @@ matrix:
        - mkdir $HOME/clang-$LLVM_VERSION
        - tar xf $LLVM_ARCHIVE_PATH -C $HOME/clang-$LLVM_VERSION --strip-components 1
 
-    - os: linux
-      compiler: gcc
-      addons:
-        apt:
-          sources: ['ubuntu-toolchain-r-test']
-          packages: ['g++-6', 'valgrind']
-      env: COMPILER=g++-6
-
     # Clang 3.5 is not able to compile the code,
     # see https://travis-ci.org/nlohmann/json/jobs/126720186