Update Zotero to 4.0.21.5

Value of `version` stanza now reused in `url`, ref. #4910.
Merge pull request #5129 from ndr-qef/zotero

Update Zotero to 4.0.21.5
install rspec; move first few tests over

refs #5080
Merge pull request #5120 from caskroom/install-rspec

install rspec; move first few tests over
Add Cask Xamarin, replacing older Xamarin Casks

These packages are: xamarin-android, xamarin-ios, and xamarin-studio.
They have been added to homebrew-versions.

Only call caveats once on Xamarin cask
rspecify scopes and download strategy tests

refs #5080
Merge pull request #5132 from caskroom/rspecify-part-1

rspecify scopes and download strategy tests
Merge pull request #5118 from fapper/xamarin

Add Cask Xamarin, replacing older Xamarin Casks
Merge pull request #5125 from jconley/luminance-hdr

add luminance-hdr 2.4.0
Seil 10.9.0
Merge pull request #5134 from NanoXD/seil

Seil 10.9.0
update CotEditor to 1.5.4
Merge pull request #5138 from vitorgalvao/fix-5137

update CotEditor to 1.5.4
Connector 2.2.3
Add Placeit version 1.7.2

Place Your App in a Stage! https://placeit.net
Merge pull request #5136 from leoj3n/connector-2.3.3

Connector 2.3.3
updated imitone (0.5.3)
Merge pull request #5139 from vinkla/placeit

Add Placeit version 1.7.2
Merge pull request #5140 from vitorgalvao/imitone

updated imitone (0.5.3)
Merge pull request #4055 from caskroom/code-of-conduct-v0.4

update CoC to 0.4