swapping slashes in gemspec file list regex to remove bundler warning

A typo in the gemspec was splitting `git ls-files` at all spaces, rather than new-lines only. This caused Bundler to warn of bad files:

```
soundmanager-rails at ... did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  ["test/dummy/public/low", "flight", "v2.mp3"] are not files
```

Swapping the line for the common `` `git ls-files`.split($/)`` fixes the problem.