Add `bundle add` command to add gems into the Gemfile

Continuing the work done to implement `bundle add GEM` from bundler/bundler#4546

To do:
- [x] Cherry-pick commit from @ognevsky
- [x] Run specs
- [x] Use Bundler.definition to resolve `last_version_number`
- [x] Pattern `bundle add` implementation after `bundle inject`
- [x] Add option to specify added gem's `group`
- [x] Add option to specify added gem's `source`
- [x] Modify existing specs & add additional specs

@RochesterinNYC

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Closes #4546

Print an outdated warning when a project is locked to an old version of Bundler

@indirect @RochesterinNYC

I'm a bit stumped.

The original issue on bundler/bundler#4707 mentions that this warning should only be printed once unless the user updates their version of Bundler, and that the warned version should be stored in `Bundler.settings[:warned_version]`.

The issue is that, AFAIK, this file, `postit_trampoline.rb` is the last place where I can access both the `installed_version` (system version) and the `running_version` (project's version) of Bundler before it completely switches over to the `running_version`.

Since `Bundler.settings` still hasn't been initialized at this point, I can't store the `running_version` in `Bundler.settings[:warned_version]`.

Right now, this warning is shown every time a `bundle` command is issued in a project directory where `installed_version != running_version`.

Is there any other way I can store the warned version from this file?

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Closes #4707

Make `bundler` and `bundle` executables have the same functionality

Closes #4725

Add spec for `bundler` executable

Installing gems without version after the ones with version for tests

`rake < 2` should be installed before `artifice`.

On `ruby 1.8.7` it fails randomly because of no specific order of hash traversal.

https://travis-ci.org/bundler/bundler/builds/141867428

Fix test failures due to Rack 2.x

- Rack 2.x requires Ruby version >= 2.2.2
- artifice doesn't support rack 2.x now.

- Rack 2.x requires Ruby version >= 2.2.2
- artifice doesn't support rack 2.x now.