Allow self-update via regular install/update commands

[indirect]

Today, upgrading RubyGems and Bundler requires "special" commands (gem update --system and bundle update --bundler, respectively).

It would improve the user experience if upgrading used the same commands to upgrade other gems. This would not only make it easier to remember how to upgrade, it would also get us more in line with other package managers, like pip (pip upgrade pip) and npm (npm install npm).

I propose we handle gem install gem and gem update gem, installing the latest version of RubyGems.

I also propose we allow bundle update bundler as a way to both 1) install the newest Bundler version (e.g. gem install bundler), and also 2) update the Bundler version used by the current Gemfile (e.g. bundle update --bundler).

Open questions:

1. Should we also handle gem install rubygems and gem update rubygems?
2. Should we also handle bundle update bundle?
3. Should we deprecate the existing update commands, or just leave them there for anyone who still remembers them?

[deivid-rodriguez]

I also propose we allow bundle update bundler as a way to both 1) install the newest Bundler version (e.g. gem install bundler), and also 2) update the Bundler version used by the current Gemfile (e.g. bundle update --bundler).

Not that bundle update --bundler already does both, so it'd be just a matter of "aliasing" bundle update bundler to what bundle update --bundler does.

[hsbt]

How gem selfupdate and bundle selfupdate?

[indirect]

@hsbt I think users will guess "gem update gem" or "gem install gem" because of other package managers (like npm, pip, etc) that use "npm install npm" or "pip update pip". Do you think users will guess selfupdate?