Remove clang-3 from CI (#807)

* Remove clang-3 from CI

* update README minimum clang version

Author: sbSteveK

.github/workflows/ci.yml

@@ -94,7 +94,6 @@ jobs:
     strategy:
       matrix:
         compiler:
-          - clang-3
           - clang-6
           - clang-8
           - clang-9

README.md

@@ -42,7 +42,7 @@ __Jump To:__
 
 ### Minimum Requirements
 * C++ 11 or higher
-    * Clang 3.9+ or GCC 4.8+ or MSVC 2015+
+    * Clang 6+ or GCC 4.8+ or MSVC 2015+
 * CMake 3.9+
 
 [Step-by-step instructions](./documents/PREREQUISITES.md)