Add RSpec Task To Rakefile.

Hi,

Not sure how you are currently running your specs, but since you already have a `Rakefile`, you can include the the rspec `rake` task in there. Then you can run all your specs by executing:

    $ rake

or

    $ rake spec

Refer the rake task [docs](https://www.relishapp.com/rspec/rspec-core/v/3-0/docs/command-line/rake-task) for a list of supported options.