Refactor rubocop
Rename for houndci
Can not open xls file when spreadsheet 0.9.9 and roo 1.13.2

     Failure/Error: subject!{ rake["heart_seed:xls"].invoke }
     LoadError:
       cannot load such file -- spreadsheet/errors
     # ./lib/heart_seed/converter.rb:71:in `open_file'

cf) https://travis-ci.org/sue445/heart_seed/jobs/33803298
Merge pull request #13 from sue445/feature/refactor_rubocop

Refactor rubocop