Update FTColumnflow.js

Hi there,

Thanks for putting together a great library! We're putting it to great use :)

We are using r.js to minify all of our sourcecode but we found that in the current state FTColumnflow is not working when using the optimizations. A small change to the library is enough to fix this problem.

However, I don't know if this conflicts with other usages of the library (it is working fine in stand alone mode here). I tried running the Buster tests, but they are failing on the master branch and on this branch as well. Might have something to do with recent updates to Buster?

Anyway, please let me know if this PR can be merged or what I need to do to make this possible!