Fix #419, FS.rename warning

#419 was caused by julia/JuliaLang@e54ff95329b9f094cdc5687c2b969f38c6920bc3, which adds a `rename` function and exports it from `FS`. Rather than extend the function, which would break compatibility with Julia 0.2, it seems easier just to avoid getting all of the `FS` module's exports in our namespace. (Eventually we may need separate branches for Julia 0.2 and 0.3, but I'd like to avoid that as long as possible.) @tanmaykm, can you confirm there's nothing else in `dataframe_blocks.jl` that needs to be changed?