Fix CircleCI error by updating the cache key

[thinkh]

Closes #297

Prerequisites:

• branch is up-to-date with the branch to be merged with, i.e. develop
• build is successful
• code is cleaned up and formatted
• tested with Firefox 52, Firefox 57+, Chrome 64+, MS Edge 16+

Summary

Following https://stackoverflow.com/a/58886629

[thinkh]

It looks like this fix only fixes the build of a single branch (see CircleCI build of #301). When pushing a new branch we would need to update the cache key again...

Edit

I forgot to use the updated CircleCI config in #301. After merging this branch it works also there as expected.