Make the script compliant with Keynote 6.1

[robertoaloi]

The following happens using the latest Keynote version:

escript: exception error: no match of right hand side value {error,eisdir}
  in function  'keynote-to-text':files/1 (src/keynote-to-text.erl, line 22)
  in call from 'keynote-to-text':main/1 (src/keynote-to-text.erl, line 12)
  in call from escript:run/2 (escript.erl, line 747)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_it/1 
  in call from init:start_em/1