`RandomNumberGenerator::result_type` should be unsigned (#1050)

`result_type` must be unsigned:
http://en.cppreference.com/w/cpp/concept/UniformRandomBitGenerator

Using a signed type causes an infinite loop working with MS Visual Studio 2017, targetting: v140, WindowsTargetPlatformVersion 10.0.15063.0, Debug, x64

include/internal/catch_test_case_registry_impl.hpp

@@ -22,7 +22,7 @@
 namespace Catch {
 
     struct RandomNumberGenerator {
-        typedef std::ptrdiff_t result_type;
+        typedef unsigned int result_type;
 
         result_type operator()( result_type n ) const { return std::rand() % n; }