Fix script always fails at the `rm package.box` line on fresh clone.

Since `set -e` is present, and a fresh cloner won't have a `package.box` file,
`rm package.box` always fails and takes the whole script with it. Add a `-f`.
Merge pull request #5 from steven-collins-omega/rm-f

Fix script always fails at the `rm package.box` line on fresh clone.