Allow scc travis-merge to search dependencies in description/comments

The `--depends-on xxx` comment used to work only via comments for inclusion
in the travis-merge. This PR:
- allows scc travis-merge to looks for inclusion filter in the description field.
- supports `--depends-on user/repo#nr` as well as `--depends-on https://github.com/user/repo/pull/xx`
- adds corresponding integration and unit tests
- adds support for dashes in username/repositories filters (cf `snoopys-sandbox`)

See discussion in openmicroscopy/openmicroscopy/pull/2150 for an example of application
/cc @ximenesuk