update CONTRIBUTING.md with state-of-the-art Guidelines

as seen in https://github.com/citrusframework/citrus/pull/940/files#diff-eca12c0a30e25b4b46522ebf89465a03ba72a03f540796c979137931d8f92055R129, the `CONTRIBUTING.md` contains old information, e.g. builds on Travis? for sure there is other obsolete content too.

task: read through it, update it!

should be done for #922.

Start by reading CONTRIBUTING.md and compare the outdated section identified in pull request #940, especially the Travis build information. Review the rest of the document for similarly obsolete content. Done means CONTRIBUTING.md reflects current project guidelines and the requested updates for #922 are addressed.

Written by the indexing model from the issue text.