add new stage-qa target: gemfiledeps

This checks whether rubygem based ports have all of their dependencies
in Gemfile(s) satisfied by what's currently installed. Sample output:

> Running Q/A tests (stage-qa)

Warning: Dependencies defined in /usr/ports/www/gitlab/work/stage/usr/local/www/gitlab/Gemfile are not satisfied

These ports could (!) be broken at runtime. Be aware: some projects
defines multiple Gemfiles and not all are used at runtime. For example
www/gitlab has two Gemfiles, but only one is used for testing and warnings
about it can be ignored.

Approved by: portmgr (mat), lifanov
Differential Revision: https://reviews.freebsd.org/D11865