Fix a few typos [ci skip]
Some tweaks to the development set-up guide

* Remove the libxml2 and libxslt part ; Nokogiri ships now with these
  libraries bundled.
* Add the missing installation instructions for either Arch Linux,
  FreeBSD or Mac OS X.
* Use the new `pkg install` syntax over `pkg_add -r` for FreeBSD 10+
  users.
* Point to the up-to-date version of PostgreSQL as for the BSD port.
* Make the platform installation instructions order consistent
  throughout the guide.
* Remove useless duplicated links to Homebrew's home.

[ci skip]
Whoops, duplicated word [ci skip]
treetop is no longer a mail (rails) dependency

see https://github.com/mikel/mail/commit/2da7c7985c221272f6451b27ab8b41e84e0a6804
[ci skip] wrap to 80 chars
[ci skip] docs asset_pipeline#CDNs
[ci skip] docs asset_pipeline CDN set up
[ci skip] docs CDN caching behavior
[ci skip] docs CDN header debugging
[ci skip] docs CDN Cache-Control behavior
[ci skip] docs can URL cache invalidation
Address comments on Gzip implementation

- don't mutate PATH_INFO in env, test
- test fallback content type matches Rack::File
- change assertion style
- make HTTP_ACCEPT_ENCODING comparison case insensitive
- return gzip path from method instead of true/false so we don't have to assume later
- don't allocate un-needed hash.

Original comments:

https://github.com/rails/rails/commit/
cfaaacd9763642e91761de54c90669a88d772e5a#commitcomment-7468728

cc @jeremy
Merge pull request #16680 from huoxito/update-dependencies

treetop is no longer a mail (rails) dependency [ci skip]
Don't ignore constraints in redirect routes

https://github.com/rails/rails/commit/402c2af55053c2f29319091ad21fd6fa6b90ee89
introduced a regression that caused any constraints added to redirect routes
to be ignored.

Fixes #16605
Merge pull request #16616 from schneems/schneems/jeremy-comments

Address comments on Gzip implementation
Merge pull request #16662 from robin850/duration-instance-of

Fix `ActiveSupport::Duration#instance_of?` to behave like `#is_a?`
[ActiveJob] Add activesupport as dependency [ci skip]
Merge pull request #16639 from agrobbin/input-placeholder-i18n

Add I18n support for `:placeholder` HTML option is passed to form fields
Merge pull request #16681 from seuros/master

[ActiveJob] Add activesupport as dependency [ci skip]
[ci skip] fix grammar in docs