Use valid type to get rid of warning message
This patch will remove following warning message in compiling.
```
rmmain.c: In function 'test_Magick_version':
rmmain.c:1705:36: warning: passing argument 1 of 'GetMagickVersion' from incompatible pointer type [-Wincompatible-pointer-types]
version_str = GetMagickVersion(&version_number);
```
@@ -1692,7 +1692,7 @@ Init_RMagick2(void)
static void
test_Magick_version(void)
{
- unsigned long version_number;
+ size_t version_number;
const char *version_str;
int x, n;
ID bypass = rb_intern("RMAGICK_BYPASS_VERSION_TEST");
