Fix parsing of .xlsx files exported from Numbers

Numbers 3.1 can export .xlsx files, but the feature has some incensistencies and bugs.

This commit handles an inconsistency where the first sheet doesn't have have an index in the XML filename ("sheet.xml"), whereas such sheet would be named "sheet1.xml" by Excel. Trying to read such files would raise the following exception:

```
undefined method `xpath' for nil:NilClass in [GEM_ROOT]/gems/roo-1.10.1/lib/roo/excelx.rb on line 459
```

This fix is inspired by a solution provided in #110 by @janis-grigaluns. Kudos!