FedEx: Attempting fix for issue #393

It looks like the issue (https://github.com/duckduckgo/zeroclickinfo-goodies/issues/393) is with the Regex not selecting `$+{to_checksum}` and `$+{extra_numbers}` correctly. According to http://images.fedex.com/us/solutions/ppe/FedEx_Ground_Label_Layout_Specification.pdf
```
For the FedEx Ground “96” bar code, digits 2 through 8 are the
package ID number; digits 9 through 15 are the shipper ID. Digits 16
through 22 are not used in the FedEx Ground “96” check digit
calculation.
```
So I've made the regex only select out 15 characters for the checksum, whereas it was `{15,20}` before. There's no test coverage for the 20 characters and I can't see any examples anywhere which would hit this.

@jagtalon do you think this is suitable or have I missed something key? Unfortunately, not being US I can't test with my own package numbers!