Unify test/vendor Gemfiles and bundler configuration.

[MikeMcQuaid]

Move all gems to vendor/bundle from vendor/bundle-standlone.

This makes a bit more sense than having e.g. rubocop in both files and getting duplicated, inconsistent Dependabot PRs with different versions.

Also:

• utils/gems: silence bundle check output (missed in Bump rubocop from 0.64.0 to 0.65.0 in /Library/Homebrew/test #5768).

• cleanup: run bundle clean (now that we cannot get Bundler to do it automatically as it'll clean up itself every time).

• Have you followed the guidelines in our Contributing document?

• Have you checked to ensure there aren't other open Pull Requests for the same change?

• Have you added an explanation of what your changes do and why you'd like us to include them?

• Have you written new tests for your changes? Here's an example.

• Have you successfully run brew style with your changes locally?

• Have you successfully run brew tests with your changes locally?

---

[sjackman]

==> Downloading https://linuxbrew.bintray.com/bottles-portable-ruby/portable-ruby-2.3.7.x86_64_linux.bottle.tar.gz‌
==> Pouring portable-ruby-2.3.7.x86_64_linux.bottle.tar.gz‌
/home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- active_support/core_ext/object/blank (LoadError)‌
	from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/global.rb:10:in `<top (required)>'
	from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:21:in `require_relative'
	from /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/brew.rb:21:in `<main>'
Service 'sut' failed to build: The command '/bin/sh -c HOMEBREW_NO_ANALYTICS=1 HOMEBREW_NO_AUTO_UPDATE=1 brew tap homebrew/core 	&& chown -R linuxbrew: /home/linuxbrew/.linuxbrew 	&& rm -rf ~/.cache' returned a non-zero code: 1‌

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