Fix asset backup file

Commit bfd01c9 removed the .json suffix from the assets_manifest_backup. This produces an error when deploying on my Ubuntu environment. Suggest changing assets_manifest_backup to assets_manifest.backup

```
DEBUG [7813d7de] Command: cd /home/deployer/discourse-production/releases/20131115183147 && ( RAILS_ENV=production cp /home/deployer/discourse-production/releases/20131115183147/public/assets/manifest* /home/deployer/discourse-production/releases/20131115183147/assets_manifest_backup )
DEBUG [7813d7de] 	cp: 
DEBUG [7813d7de] 	target `/home/deployer/discourse-production/releases/20131115183147/assets_manifest_backup' is not a directory
```