rstudio/shiny-examples — pull request #5 by knbknb (2014-09-02 21:08:38+00:00)
Title: 1-line bugfix for newer versions of "tm" package > v0.60

Avoid Error message
" Warning in mclapply(unname(content(x)), termFreq, control) :
all scheduled cores encountered errors in user code
Error in UseMethod("meta", x) :
no applicable method for 'meta' applied to an object of class "character"
"

The latest version of tm (0.60) made it so you can't use functions with tm_map that operate on simple character values any more. So the problem is your tolower() step since that isn't an "canonical" transformation (See getTransformations()).
see: http://stackoverflow.com/a/24771621/202553