call grunt.file-delete with {force: true} when deleting intermediate files

Calls to `grunt.file.delete` blow up when deleting files outside of the Grunt base directory.  This is causing imagemin to fail on my project has the following directory structure:
```
- src
  |-- Gruntfile.js
  |-- foo.png
- build
  |-- foo.png
```
The `grunt.file.delete` safeguard seems reasonable for a task that deletes files that it didn't create.  It doesn't seem helpful for temporary files that are created and deleted all within the lifecycle of a single task invocation.  Pass the `force` flag to `grunt.file.delete` to allow the file deletion even if it's outside the Grunt base directory.