Use the preferred `bundle cache` everywhere, but leave package as an
alias.

Remove duplicated tests.

I don't think we need to keep this information in our docs. Users should
read docs for the version they're using and the history of bundler
distract her from the point of the docs, in my opinion.

7385: Remove taint usage on Ruby 2.7+ r=hsbt a=jeremyevans

### What was the end-user problem that led to this PR?

Ruby 2.7 is deprecating taint support. 

See https://bugs.ruby-lang.org/issues/16131 for details.

### What was your diagnosis of the problem?

Bundler still uses `untaint` in a few places, which will cause deprecation warnings on Ruby 2.7.

### What is your fix for the problem, implemented in this PR?

`s/\.untaint/.tap{|x| x.untaint if RUBY_VERSION < "2.7" }/g`

### Why did you choose this fix out of the possible options?

It seemed simplest.


Co-authored-by: Jeremy Evans <code@jeremyevans.net>

7379: Switch to double quotes to match rest of file r=deivid-rodriguez a=jonathanhefner

### What was your diagnosis of the problem?

It is common to prefer double quotes, unless single quotes are needed.  It is also generally preferred to use a consistent quote style (whether double quotes or single quotes).

### What is your fix for the problem, implemented in this PR?

Switch the anomalous single quotes to double quotes.


Co-authored-by: Jonathan Hefner <jonathan@hefner.pro>

Ruby 2.7 deprecates taint and it no longer has an effect.
This attempts to leave the behavior the same on older
Ruby versions, but avoid the use of untaint on Ruby 2.7+.

See https://bugs.ruby-lang.org/issues/16131 for details.

4474: Improve readability of outdated r=deivid-rodriguez a=tlynam

Haven't fixed tests since wanted buy-in.

Before:

```
Outdated gems included in the bundle:
  * active_model_serializers (newest 0.9.5, installed 0.8.3, requested ~> 0.8.0) in group "default"
  * activerecord-import (newest 0.13.0, installed 0.10.0) in group "default"
  * addressable (newest 2.4.0, installed 2.3.5, requested = 2.3.5) in group "default"
  * airbrake (newest 5.2.3, installed 5.1.0) in group "default"
  * bootstrap-sass (newest 3.3.6, installed 3.3.5.1) in group "default"
  * capistrano (newest 3.5.0, installed 3.4.0, requested ~> 3.4) in group "development"
  * capistrano-rails (newest 1.1.6, installed 1.1.3, requested ~> 1.1.1) in group "development"
```

After:

```
--pretty
Outdated gems included in the bundle:
Gem Name                New        Installed  Requested  Groups
airbrake                5.3.0      5.0.3                 default
better_errors           2.1.1      1.0.1      ~> 1.0.1   development
bootstrap-sass          3.3.6      3.3.0.1    ~> 3.3.0.1 default
factory_girl_rails      4.7.0      4.4.1                 test, development

--pretty --verbose
Outdated gems included in the bundle:
Gem Name                New        Installed  Requested  Groups            Load Path
airbrake                5.3.0      5.0.3                 default           /Users/todd/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/specifications/airbrake-5.3.0.gemspec
better_errors           2.1.1      1.0.1      ~> 1.0.1   development       /Users/todd/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/specifications/better_errors-2.1.1.gemspec
bootstrap-sass          3.3.6      3.3.0.1    ~> 3.3.0.1 default           /Users/todd/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/specifications/bootstrap-sass-3.3.6.gemspec
factory_girl            4.7.0      4.4.0                                   /Users/todd/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/specifications/factory_girl-4.7.0.gemspec
factory_girl_rails      4.7.0      4.4.1                 test, development /Users/todd/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/specifications/factory_girl_rails-4.7.0.gemspec
```


Co-authored-by: Todd Lynam <tlynam@gmail.com>
Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>