Add gem constraints

I added a bunch of gem constraints to each gem in the Gemfile which corresponds to the current major.minor.x version locked in Gemfile.lock.

This gives the flexibility of being able to run bundle update and receive patch updates only, which if semver semantics are followed by the gem author(s) should only include fixes and no breaking changes.

I also bumped Ruby to 2.5.1 (from 2.5.0) as that's the most current 2.5.x release.

--

Update gems using bundle update.

• Run the image:bootstrap manual job right after this MR is merged