Allow to build gems with @opsbots

## Installation

- Add an admin to the lita configuration
```ruby
  config.robot.admins = "209293_1471648@chat.hipchat.com"
```
- Add people to the logstash group with
```
@botmention auth add "Pier-Hugues Pellerin" logstash
```
- Publish a gem with 
```
@botmention publish https://github.com/ph/test-opsbots
```
- Make sure you have valid rubygems credentials ` ~/.gem/credentials`

## Caveats

- Doesn't work with `rbenv` yet.

is this right? `sh -e exit ` runs 'exit' as a script; did you mean `sh -c 'exit 1'` ?