Only run rails 4 tests a single time from the command line

[zmoazeni]

I don't know why sprockets wants to concatonate all files in rails 4
but this will explicitly write an empty file for jasmine-specs.js
which stops the duplicate test runs.

This is extremely hackish, but I can't find a way around this.

Fixes #28

[zmoazeni]

By the way, this is a lolfix.

If you have a jasmine.yml like:

spec_files:
  - "spec.js"

and a javascripts/spec.js like:

//= require baz_spec
//= require foo_spec

The tests will still run twice.