version bump 0.5.10: numbers iOS fix

Most writers write sheets to

xl/worksheets/sheetN.xml (where N is 1-indexed)

Numbers for iOS apparently writes to

xl/worksheets/sheet.xml (for first sheet)
xl/worksheets/sheetN.xml (N>1, 0-indexed -- sheet1.xml corresponds to 2nd sheet)

The right thing to do is to dig into workbook rels, but that's for another day