Repo bootstrap fix

## /opt/boxen/repo initialization is broken

One of the config values that we pass to Heroku app is `REPOSITORY=our-org/our-boxen`. But it doesn't get passed to the installation script that runs on a target machine.
So installation process encounters errors:

```
Notice: /Stage[main]/Boxen::Repo/Exec[clone /opt/boxen/repo]/returns: Initialized empty Git repository in /opt/boxen/repo/.git/
Notice: /Stage[main]/Boxen::Repo/Exec[clone /opt/boxen/repo]/returns: fatal: https://github.com/undef/info/refs?service=git-upload-pack not found: did you run git update-server-info on the server?
Error: git init && git remote add origin https://github.com/undef && git fetch -q origin && git reset --hard origin/master returned 128 instead of one of [0]
Error: /Stage[main]/Boxen::Repo/Exec[clone /opt/boxen/repo]/returns: change from notrun to 0 failed: git init && git remote add origin https://github.com/undef && git fetch -q origin && git reset --hard origin/master returned 128 instead of one of [0]
Notice: /Stage[main]/Boxen::Bin/File[/opt/boxen/bin/boxen]: Dependency Exec[clone /opt/boxen/repo] has failures: true
Warning: /Stage[main]/Boxen::Bin/File[/opt/boxen/bin/boxen]: Skipping because of failed dependencies
```

## The Fix

It's simple: set `BOXEN_REPO_NAME` environment var just before running `script/boxen`. See 37aaee0.