postmodern/chruby — pull request #101 by dejan (2013-02-20 11:11:06+00:00)
Title: 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 empty, appending will cause syntax error.

```
-bash: PROMPT_COMMAND: line 4: syntax error near unexpected token `;'
-bash: PROMPT_COMMAND: line 4: `; chruby_auto'
```