Log Message:
libxlsxwriter: Update to 0.91

Changes in 0.91:
 - Fix to missing MD5 linkage in Cocoapod file.

Changes in 0.90:
 - Fix to avoid duplicate images being copied to an libxlsxwriter
   file. Excel uses an optimization where it only stores one copy of
   a repeated/duplicate image in a workbook. Libxlsxwriter didn't do
   this which meant that the file size would increase when then was a
   large number of repeated images. This release fixes that issue and
   replicates Excel's behavior.