spec_dir accept array of directories

Hi,

`spec_dir` configuration in jasmine.yml to accept array of directories. Such as:

```
spec_dir:
  - spec/javascripts
  - ../engine1/spec/javascripts
  - ../engine2/spec/javascripts
```

Very useful for a large project split into engines, when each engine contain javascript files and their associated tests.

Note: If you are to test this pull request you may have to `rake assets:clobber` due to the assets pipeline caching

Thanks for your feedback