Use YAML::SyntaxError since ParseError does not exist under Psyck Yaml

implementations.

Running the synapse binary gives:
```
> synapse
/Users/jbellegarde/src/synapse/bin/synapse:40:in `rescue in parseconfig': uninitialized constant Psych::ParseError (NameError)
        from /Users/jbellegarde/src/synapse/bin/synapse:34:in `parseconfig'
        from /Users/jbellegarde/src/synapse/bin/synapse:46:in `<top (required)>'
        from .bin/synapse:16:in `load'
        from .bin/synapse:16:in `<main>'

```

YAML is an alias for Psych. Psych has SyntaxError instead of  the Sick's ParseError.