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Update QUnit to v1.18.0.

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Remove defunct responsive-(in)visibility mixins

inline the one remaining usage into .hidden-print's definition
rm now-empty scss/_utilities-responsive.scss file

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Clear out non-v4 docs translations

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Bump year in jumbotron example

Rebased & tweaked version of #17272

fixes #17097: Go back to using jQuery's text and html methods since innerText is nonstandard and not present in Firefox

Closes #17272 by merging a tweaked version of it.

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Add babel:dev to Sauce Grunt subtasks so we don't test stale JS

Refs #17328

Add babel:dev to dist-js so `grunt test` doesn't use stale JS

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Closes #17319 by merging it.

rm broken bundle update task & defunct custom RubyGem caching scheme

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[v4] Fix for Meteor package: removed font files

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v4 Travis Build

sets up infra to properly build Bootstrap

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Fixes the _image mixing by prefixing the parameters with $

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Explicitly document existing de facto policy of supporting ESR Firefox