Setting PROMPT_COMMAND when blank in auto.sh

Issue was mentioned here: https://github.com/postmodern/chruby/issues/99#issuecomment-13823660

When PROMPT_COMMAND is blank, appending will cause syntax error.

```
-bash: PROMPT_COMMAND: line 4: syntax error near unexpected token `;'
-bash: PROMPT_COMMAND: line 4: `; chruby_auto'
```