Recently some people, including myself, ran into an issue with running SMW master via Composer.
One can specify that master should be used using "dev-master". This does however result into a conflict if you also have an extension to SMW, such as Semantic Result Formats, installed via Composer. The conflict is between that extension specifying they need a version of SMW within a specific range, ie 1.9.*, and dev-master not being in this range. One can change "dev-master" to "@dev" or "*", which fixes the conflict, though then gives you the most recent release of SMW, rather than master.
We now have a branch alias for master named "1.9.x-dev", which can be used to get master without running into the above described conflict. This can be used immediately and has already been done on SMW wiki: https://semantic-mediawiki.org/wiki/Special:Version