Update bundler version

[andyundso]

There are two common places in our projects where we can find bundler versions:

• In the Gemfile.lock
• In the Dockerfile, where we install bundler.

This PR:

• Adds a regex manager. that finds the current bundler version for a Gemfile and updates it. Usually, you do this by executing bundle update --bundler, but I do not expect any troubles with directly replacing the version, at least while the lockfile format does not change.
• Adds in the regexManagers:dockerfileVersions preset. Not sure if a preset can extend another preset, will have to see.

[Zinggi]

Not sure how safe the regex matcher is to update bundler, but let's try.
Because now renovate will update the lockfile with potentially a different bundler version, right?