Add CI to check current master works

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/885)
In GitLab by [[Gitlab user @jjardon]](https://gitlab.com/jjardon) on Jan 28, 2019, 15:29

So we do not regress when the dependencies in requirements/requirements.in are not correct

Our installation recommended method to install from source is `pip3 install --user .` (https://buildstream.build/source_install.html#install_git), so we need to be sure this works

See https://gitlab.com/BuildStream/buildstream/issues/884

Start with requirements/requirements.in and the source-install instructions linked in the issue, especially the recommended `pip3 install --user .` path. Add CI coverage that installs the current master using that method and confirms it works when the dependency requirements are incorrect; use issue 884 for related context.

Written by the indexing model from the issue text.