Update ruby-spreadsheet to 0.8.1.

=== 0.8.1 / 18.02.2013

* Updated Manifest.txt to include lib/spreadsheet/excel/rgb.rb

=== 0.8.0 / 18.02.2013

* Adding support for converting color palette values to RGB values (not vice-versa..yet)
* by https://github.com/dancaugherty/spreadsheet/compare/master...rgb

=== 0.7.9 / 06.02.2013

Author: Eugeniy Belyaev (zhekanax)

* You can merge if you are interested in perl-like Workbook.set_custom_color
  implementation. I know it is not really a proper way to deal with custom colors, but
  nevertheless it makes it possible.
* https://github.com/zdavatz/spreadsheet/pull/27

=== 0.7.8 / 06.02.2013

Author: Kenichi Kamiya <kachick1@gmail.com>
Date:  Wed Feb 6 11:23:35 2013 +0900

* Link to Travis CI on README
* Remove warnings "assigned but unused variable" in test
* Remove warnings "assigned but unused variable"
* Enable $VERBOSE flag when running test

(obache)