Increase project import timeout from 4 minutes to 15 minutes

Many users were experiencing timeouts when we only allowed
4 minutes before a timeout. A 15 minute timeout is more
than reasonable and prevents us from having to add a
configuration for this.

Fixes gitlab-org/gitlab-ee#246

## Import screenshot

![Screen_Shot_2016-02-04_at_1.45.20_PM](/uploads/d9d71ec953afae31e6fbe54a8932b945/Screen_Shot_2016-02-04_at_1.45.20_PM.png)


See merge request !2717