Support to print sha1 on bottling with --rb.

This PR is related on #74.
There doesn't output sha1 hash for generated bottles like this:
```
./hello-2.9.linux.bottle.tar.gz
  bottle do
    prefix "/home/azureuser/.linuxbrew"
    cellar "/home/azureuser/.linuxbrew/Cellar"
  end
```

After applied my patch, it'll show like this:
```
./hello-2.9.linux.bottle.tar.gz
  bottle do
    prefix "/home/azureuser/.linuxbrew"
    cellar "/home/azureuser/.linuxbrew/Cellar"
    sha1 "88aaf685c1bd21927ec94f28aae8ed46c6e4df50" => :linux
  end
```