Remove use of global variables to get Regexp captures in Subtitle#shift

Since `$1` and `$2` are not completely compatible with MacRuby, our uses changed to the similar `result[0]` and `result[1]` form.

I didn't added any test cases since this case (and the entire suite, which I am planning to support it for this implementation of Ruby) should be tested against MacRuby itself, not just a single variation between MRI and MacRuby.