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_random_number_generator.h

@@ -20,7 +20,7 @@ namespace Catch {
     unsigned int rngSeed();
 
     struct RandomNumberGenerator {
-        using result_type = std::ptrdiff_t;
+        using result_type = unsigned int;
 
         static constexpr result_type (min)() { return 0; }
         static constexpr result_type (max)() { return 1000000; }