Fix Listen implementation so master is compatible with 1.8.7

- remove Ruby >= 1.9.x hash syntax
- no newlines between
- allow `listen ~> 1.3`. [changelog from v1.3.0 to v1.3.1](https://github.com/guard/listen/compare/v1.3.0...v1.3.1). In case new versions are released that are critical in v1.x, it'd be nice to allow it.