fix coffee:test task

Source and destination directories were swapped and generated file extension was missing, which prevented test files written in Coffeescript from being compiled properly

### Step to reproduce:
(assume yo + generator-backbone already installed)

* yo backbone
* 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