Version 15 sheet coordinates are incorrect

[bjhijmans]

The version 15 emoji data sheets added an extra row of emojis

https://github.com/iamcal/emoji-data/blob/master/sheets-indexed-256/sheet_twitter_32_indexed_256.png

However, the coordinates were not updated in this package. So e.g. it lists the brazil flag emoji flag-br as coordinate x=1, y=1. However, in the sheet I linked the brazilian flag is actually at x=1, y=0 now.