Make non-ancient version of GCC explicit in documentation (it means 4.8+) (#575)

diff --git a/docs/intro.rst b/docs/intro.rst
index 5455dd2..2149c18 100644
--- a/docs/intro.rst
+++ b/docs/intro.rst
@@ -90,6 +90,6 @@
 *******************
 
 1. Clang/LLVM (any non-ancient version with C++11 support)
-2. GCC (any non-ancient version with C++11 support)
+2. GCC 4.8 or newer
 3. Microsoft Visual Studio 2015 or newer
 4. Intel C++ compiler v15 or newer