Fix undefined behavior for right-shift(#531)

The result of a right-shift of a signed negative number is
implementation-dependent.
cf. https://msdn.microsoft.com/en-us//library/336xbhcz.aspx

Fix #519
Fix #521

include/LightGBM/utils/random.h

@@ -103,7 +103,7 @@ private:
     return x & 0x7FFFFFF;
   }
 
-  int x = 123456789;
+  unsigned int x = 123456789;
 };