superenv: more 10.11 clock_gettime Autotools fixes

some build systems check ac_cv_search_clock_gettime instead of
ac_cv_func_clock_gettime so the former should also be set to "no"

libev.m4 unsets ac_cv_func_clock_gettime, but if ac_have_clock_syscall
is defined, it will leave ac_cv_func_clock_gettime alone:

  http://cvs.schmorp.de/libev/libev.m4?view=markup#l23

download_strategy: silence unzip and unrar

update.sh: hide GITHUB_HEADERS missing output.

We silenced tar in 5e3a26bf. It makes sense to make unzip and unrar
output comparably succinct.

Will only happen once but could be confusing anyway.

Rubocop style fixups

RuboCop changes.

bump-formula-pr: enhancements

intended to be used in combination with --dry-run for a not-so-dry run

the expected file modifications are made, but no git actions are taken

and add explicit version when not preexisting

--version=1.2.3 option to set `version "1.2.3"`

Note that --version=0 will remove a no-longer-needed explicit version

info: print requirements

RuboCop: MacOS

add pcmpl-homebrew for homebrew emacs plugin

Speed up `parallel_tests` by splitting integration tests.

Fix documentation for --caskroom default

fixtures: add test.m4a

doctor: remove outdated Homebrew check.

ARGV: kegs: error on empty arguments

test.m4a contains an AAC LC stream inside an M4A (MPEG-4 Part 14)
container. It is generated by FFmpeg 3.1.3 from test.wav:

    ffmpeg -y -i test.wav -c:a aac -profile:a aac_low -b:a 32k test.m4a

test.m4a facilitates testing utilities or libraries that work with the
AAC codec or the M4A container, e.g., alac.

update.sh: don't restore branch to stable.

This is no longer relevant in the bright, new tagged future.

Fixes #1151.

Only run for Homebrew developers so assume they don’t want to switch
back to an old stable branch.

Fixes #1141.

update.sh: use HOMEBREW_UPDATE_TO_TAG.