Merge branch 'develop' of https://github.com/nlohmann/json into develop

.github/SECURITY.md

@@ -0,0 +1,5 @@
+# Security Policy
+
+## Reporting a Vulnerability
+
+Usually, all issues are tracked publicly on [GitHub](https://github.com/nlohmann/json/issues). If you want to make a private report (e.g., for a vulnerability or to attach an example that is not meant to be published), please send an email to <mail@nlohmann.me>. You can use [this key](https://keybase.io/nlohmann/pgp_keys.asc?fingerprint=797167ae41c0a6d9232e48457f3cea63ae251b69) for encryption.

.travis.yml

@@ -204,13 +204,21 @@ matrix:
 
   - os: linux
     compiler: gcc
-    env:
-      - COMPILER=g++-8
-      - CXXFLAGS=-std=c++17
+    env: COMPILER=g++-9
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
-        packages: ['g++-8', 'ninja-build']
+        packages: ['g++-9', 'ninja-build']
+
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER=g++-9
+      - CXXFLAGS=-std=c++2a
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: ['g++-9', 'ninja-build']
 
   # Linux / Clang