Bundler is actually a runtime dependency

In `lib/gemrat/runner` the bundle command is called via shelling out.
It's definitely a runtime dependency.
Dependencies: Set the versions to the current version

According to #10, the gem maintainer @DruRly prefers to set
the version to a specific version instead of using "~>".
So I did it this way :wink:
Runner: Don't shell out, call Bundler::CLI directly