Update io.jl

I am not sure if I am catching what the original purpose of this if/else statement was but at least now I can correctly write out dataframes that have strings in them.
Merge pull request #340 from tanmaykm/master

importing Base.Sort after JuliaLang/julia#3931
Merge pull request #339 from dreiss-isb/patch-1

Update io.jl
Test that string columns print correctly