Update tasks/usemin.js

Line 152
Change: I removed [0] from the filepath response.
Rationale: I am running multiple builds for different locales in a loop, restricting the filepath response results in the 1st file rev'd in the loop persisting and being copied to all corresponding index.html's.
Notes: Please can you let me know if this will cause any damage, or whether there is an existing way to achieve the goal.
changed usemin directory name to staging
Merge branch 'master' into dtuk