Teach brew to recognize gcc-HEAD as valid compiler

Building gcc with --HEAD results in most of the executables not having a
version suffix, e.g. Building/installing gcc 6 would result in gcc-6,
g++-6, etc. being installed, while building/installing gcc --HEAD would
result in gcc-, g++-, etc. being installed.

The lack of a version suffix prevented brew from recognizing a valid gcc
install, resulting in brew instructing users to install gcc before
building certain formulae even though gcc is installed.

A patch to the gcc formula makes the version number for --HEAD builds
the major version number of the stable version + 1 (7 at this time).
This patch teaches brew to recognize current --HEAD builds as valid
compilers.

cmd/commands: always show developer commands

xcode: 8.0 is no longer prerelease

pkg-config: update zlib for macOS Sierra

Fix cask installation from untapped Tap not working.

test-bot: fix test default formula handling.

Move the test default formula handling to after all the other formulae
additions are so it'll only be added if no formulae are detected
already in a PR.

test-bot: don't run Homebrew tests for taps PRs.

Check the repository directly rather than @test_default_formula.

Fix Library/Homebrew/extend RuboCop warnings

test-bot: default to building testbottest.

This provides an integration test for `test-bot.rb` by performing
compilation of an actual formula.

Minimal C program and Makefile used for testing Homebrew.

test-bot: fix summary

keg: make guile's site-dir persistent

test-bot: fix bottling non-testing jobs.

Need to handle when the `BOT_PARAMS` are `nil`.

test-bot: fix multiline regex.

bottle: remove debug statements.

test-bot: fix regex placement.

test-bot: handle more Jenkins configurations.

Fix Library/Homebrew/dev-cmd RuboCop warnings

Fix Library/Homebrew/utils RuboCop warnings

Fix Library/Homebrew/test RuboCop warnings