fix coffee:test task

**Step to reproduce:**
(assume yo + grunt-cli + bower installed globally)
* yo webapp
* npm install
* delete test/spec/test.js
* create test/spec/test.coffee:

    describe "Give it some context", ->

* grunt coffee:test

**Expected result:**

    Running "coffee:test" (coffee) task
    File .tmp/spec/test.js created.
    Done, without errors.

**Actual result:**

    Running "coffee:test" (coffee) task
    Done, without errors.

No .tmp directory created