1. Running `brew linkage some_package` does not set the cache.
2. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `true` should build the cache.
3. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `false` should use the cache.

Lazily load db of type `DBM` instance variable for `DatabaseCache` so the corresponding database file isn't created in the `.use` block for a `DatabaseCache`.

Made `DatabaseCache.new` private, and changes instances in code that call it to use `DatabaseCache.use` instead.

Use cache if `HOMEBREW_LINKAGE_CACHE` exists in `formula_cellar_checks` and `formula`. Make functions private in `LinkageChecker`.

portable-ruby 2.3.3_2

Now with a working DBM on Linux.

Update issue templates

Use the new issue template improvements: https://blog.github.com/2018-05-02-issue-template-improvements/

This seems like it could fix the GitHub Pages build.

Merge pull request #4139 from Homebrew/dependabot/bundler/Library/Homebrew/test/parallel_tests-2.21.3

Bump parallel_tests from 2.21.2 to 2.21.3 in /Library/Homebrew/test

docs: fix Jekyll theme usage.

Bumps [parallel_tests](https://github.com/grosser/parallel_tests) from 2.21.2 to 2.21.3.
- [Release notes](https://github.com/grosser/parallel_tests/releases)
- [Commits](https://github.com/grosser/parallel_tests/compare/v2.21.2...v2.21.3

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bump rspec-retry from 0.5.6 to 0.5.7 in /Library/Homebrew/test

Bump simplecov from 0.15.1 to 0.16.1 in /Library/Homebrew/test

Need to use `remote_theme` not `theme` for this to work on GitHub Pages.
Handily, though, this removes the need for making and uploading a gem.

Bumps [simplecov](https://github.com/colszowka/simplecov) from 0.15.1 to 0.16.1.
- [Release notes](https://github.com/colszowka/simplecov/releases)
- [Changelog](https://github.com/colszowka/simplecov/blob/master/CHANGELOG.md)
- [Commits](https://github.com/colszowka/simplecov/compare/v0.15.1...v0.16.1

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [rspec-retry](https://github.com/noredink/rspec-retry) from 0.5.6 to 0.5.7.
- [Release notes](https://github.com/noredink/rspec-retry/releases)
- [Changelog](https://github.com/NoRedInk/rspec-retry/blob/master/changelog.md)
- [Commits](https://github.com/noredink/rspec-retry/compare/v0.5.6...v0.5.7

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bump github-pages from 158 to 183 in /docs

Bumps [github-pages](https://github.com/github/pages-gem) from 158 to 183.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](https://github.com/github/pages-gem/compare/v158...v183

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Use Homebrew’s Jekyll theme

These means we can remove a bunch of the HTML from this repository and
move it to https://github.com/homebrew/jekyll-theme-homebrew.

bump-formula-pr: fix tag/revision argument.

Can't do `@args[hash_type]` if `hash_type` is `nil`.

Remove completions for (un)linkapps commands

These were deleted in cffa5a98.

bump-formula-pr: Use Parser to parse args

Support GCC 8