Fix test suite. Use StreamTrackIds instead of nonexisting FileTrackIds.

Hey,

I was getting that error when trying to run specs with `rspec spec/`:
```
tomaszgieres@kajtek:~/Projects/spotify-to-mp3(master⚡) » rspec spec
/Users/tomaszgieres/Projects/spotify-to-mp3/spec/app/file_track_ids_spec.rb:1:in `require': cannot load such file -- spotify_to_mp3/app/file_track_ids (LoadError)
        from /Users/tomaszgieres/Projects/spotify-to-mp3/spec/app/file_track_ids_spec.rb:1:in `<top (required)>'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `each'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load_spec_files'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/command_line.rb:22:in `run'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:80:in `run'
        from /Users/tomaszgieres/.rvm/gems/ruby-1.9.3-p448/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:17:in `block in autorun'
```

The spec suite runs correctly now and everything is green.