Fix Ruby unused variable warning

This patch fixes the following warning:

```shell
monetize-1.13.0/lib/monetize/parser.rb:129: warning: assigned but unused variable - possible_major
```

Author: viralpraxis

lib/monetize/parser.rb

@@ -127,7 +127,7 @@ def minor_has_correct_dp_for_currency_subunit?(minor, currency)
 
     def extract_major_minor_with_single_delimiter(num, currency, delimiter)
       if expect_whole_subunits?
-        possible_major, possible_minor = split_major_minor(num, delimiter)
+        _possible_major, possible_minor = split_major_minor(num, delimiter)
         if minor_has_correct_dp_for_currency_subunit?(possible_minor, currency)
           split_major_minor(num, delimiter)
         else