Specifies variable in grunt-compare-size options.cache to match .gitignore file

Just noticed in my internal build and pulled the docs from https://github.com/rwldrn/grunt-compare-size#documentation and the project's .gitignore

Build works as expected when modified:

```bash
# Current upstream/master
$ grunt compare_size
# output
$ git status -s
?? .sizecache.json


# Pull Request
$ grunt compare_size
# output
$ git status -s
$ 
```