Merge pull request #580 from shtirlic/fix_warns

Fix compile warnings in mirb

tools/mirb/mirb.c

@@ -143,7 +143,7 @@ print_cmdline(int code_block_open)
 int
 main(void)
 {
-  char last_char;
+  int last_char;
   char ruby_code[1024] = { 0 };
   char last_code_line[1024] = { 0 };
   int char_index;