This adds various "doctor" tasks that can be used for troubleshooting. To see
all the doctor output, run e.g. `cap production doctor`. This will print a
report like this:
```
Environment
Ruby ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]
Rubygems 2.6.2
Bundler 1.11.2
Command cap production doctor
Gems
capistrano 3.4.0
airbrussh 1.0.1
rake 10.5.0 (update available)
sshkit 1.9.0
capistrano-bundler 1.1.4
capistrano-rails 1.1.6
Variables
:application "myapp"
:assets_prefix "assets"
:assets_roles [:web]
:branch "master"
... etc.
```
To obtain the variables information in particular, code has been added to audit
the setting and fetching of variables. Variables set by Capistrano itself and
its plugins are whitelisted, but others are "untrusted". If a variable is
untrusted and it seems like it is never used, then `doctor:variables` will print
a warning (include source location) for that variable name, like this:
```
:copy_strategy is not a recognized Capistrano setting (config/deploy.rb:14)
```
Finally, the RubyGems API is used to check the remote gem repository to see if
any newer versions of Capistrano gems are available (this is gracefully skipped
if there is no network connection). Any outdated gems will be indicated in the
`doctor:gems` output.
