fix MaxRss out of bounds hhvm don't stop problem

When more than 6g not stop hhvm,beacause function Process::GetProcessRSS return type is int;
if Process::GetProcessRSS return value than 1024*6, Process::GetProcessRSS * 1024 * 1024 in checkMemery , it return value is 0;
So is never greater than 0 MaxRSS;

e.g:

	#include <stdio.h>
	int main () {
	   int a = 1024 * 1024 * 1024 * 12;
	   printf ("%d\n",a) ;
	   a = 1024 * 1024 *  12;
	   printf ("%d\n",a) ;
	   return 0;
	}

print result:
0
12582912

we see first var a have out of bounds.

so we can modify Process::GetProcessRSS return type to int64_t solve this problem.

I have tested this modify way.