How to update Coveralls badge image to reflect actual coverage percentage?

I use a Coveralls badge on the README and documentation page to display the code coverage for my project. The badge image often displays an out-of-date percentage. Clearing my browser's cache helps, but that often still gets me an out-of-date percentage image as well, although it might be more up-to-date than the previously displayed badge.

For example, at the time of this writing the badge displays the correct percentage, currently at 91%, on the documentation page at readthedocs.io, but the badge on my GitHub README displays 89%.

Can anyone suggest how I can manually force the badge image to update on the GitHub README, i.e. how to manually pull/force an updated image from Coveralls that shows the correct percentage? Thanks in advance for your help.