Hello Simon
This is no issue regarding the bundle, it's just a fix on the readme files: the TM_FLEX_USE_FCSH env var should actually be "true", not anything. (that true is without quotes, just the 4 letters, true)
I looked into the build.rb and there is no strict equality there, so I don't know why the error came up.
Don't know also if this applies to anyone else, but it's the second time it happens for me and it took 3+ hours to find where. I even tried to reinstall ruby...
all the best, and thanks for the great work