formula_installer: correctly handle build dep reqs.

download_strategy: use tar xf.

The flags and separate `-` aren't required on 10.5 which is the oldest
version of macOS we support (and it looks nicer this way).

When a build dependency has a requirement it should be ignored when
checking for valid requirements. This requires calculating the
dependencies for a formula again and checking them when calculating
which requirements should be pruned.

extend/ENV: support CX11 for LLVM Clang.

Fix some checks for `:clang` which should match for either `:clang` or
`:llvm_clang`. Note that's not every check.

Download refactoring

travis.yml: update xcode to 9.4

bottle_block_cop: remove

Refactor staging step for download strategies.

audit: disallow unstable specs for versioned formulae

Refactor and fix VCS download strategies.

config: handle HOMEBREW_TEMP being unset.

vendor: ruby-macho 2.0

This can happen when updating from a previous version of Homebrew.

cask audit: check for devmate and hockeyapp appcasts

 Update `Gemfile.lock` and use `Pathname#/`.

Cleanup HOMEBREW_TEMP handling

- Ensure that `HOMEBREW_TEMP` is only displayed in `brew config` when
  it's non-default.
- Attempt to create a missing `HOMEBREW_TEMP` directory rather than
  failing to `realpath`. Note this will still fail on permissions errors
  which is to be expected.

audit: exact match throttled formula names

brew.sh: Don't allow system tmp dirs as prefixes

components_redundancy: audit `stable do` without a head or devel spec

audit: exempt versioned formulae from new formula audit

Show tapped casks.

Bump github-pages from 186 to 187 in /docs