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.
Add integration test for `scc travis-merge` command

Integration tests create 2 branches and PRs and test scc travis-merge on the
first branch. Inlcuded tests are:
- presence of `TRAVIS_PULL_REQUEST` environment variable
- dependency comments in the PR body or comments
Add support for URL parsing in scc travis-merge
Add integration test for URL-based depedency inclusion
Add support for username/repositories with dash, e.g. snoopys-sandbox
Handle PR filter combination directly in find_candidates()
Merge pull request #148 from sbesson/travismerge_parse

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