Add Bundler as a runtime dependency

[Manfred]

By adding bundler as a runtime dependency we can require Bundler without having to bundle exec in the target application. This makes perf_check / work instead of bundle exec perf_check /. It's useful for when you want to use PerfCheck without adding it to the bundle of the target application.

[Manfred]

It seems we are stuck in a catch-22. We want to use --frozen to prevent Bundler from changing the lockfile, but that also writes bundler config and causes issues when trying do subsequent bundle installs. I'm going to think about this for a while.

[Manfred]

Because PerfCheck now requires bundler I had to re-create Gemfile.lock in the test application bundles.