Promote stackprof into a production gem

stackprof and ruby-prof are complementary tools: the former is a
sampling profiler, and the latter is a tracing profiler. It would be
quite helpful to have stackprof available to profile endpoints in
production or on Omnibus.

Diff between v0.2.13 and v0.2.15:
https://github.com/tmm1/stackprof/compare/v0.2.13..v0.2.15

Closes https://gitlab.com/gitlab-org/gitlab/issues/199994