tests: add cmd/audit unit tests

list: note that `ls` command is used.

Enable vendored Ruby 2.0.

brew: sanitize BASH_ENV

Test that sandbox does not complain about bogus .pyc errors and does
complain about other failures.

Closes #684.

These are never fatal and often confusing.

Fixes #683.

update-test: tweak master not updated check.

Loosen this a bit; we don't necessarily expect the end commit is the one
we're looking for, just that it has changed from the start commit (i.e.
some sort of update has occurred).

Addresses some false negatives on `master` branch merges that weren't
present on the PR commits.

And arguments can be passed through to it.

Tweak produced out of conversation in
https://github.com/Homebrew/brew-evolution/pull/8.

Ship Homebrew auto-update support.

search: remove thread-unsafe Hash cache.

Silence auditing formula revisions deprecations

update: add --force argument.

tests: refactor FormulaTextTests in test_cmd_audit, add assertion to test_simple_valid_formula

install_dependencies: Do not truncate dependencies

pull: non-core tap support for Homebrew Testing jobs

versions should be initialized even if formula is not installed

`test-bot --ci-testing` supports a `--tap` option for non-core taps, so
`brew pull` should too when pulling Homebrew Testing jobs.

Swats away this annoying warning when the test is skipped due to GPG being
unavailable:

```
  1) Skipped:
GpgTest#test_create_test_key:
GPG Unavailable

Error:
GpgTest#test_create_test_key:
NoMethodError: undefined method `rmtree' for nil:NilClass
    /usr/local/Library/Homebrew/test/test_gpg.rb:11:in `teardown'
```

Closes #675.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Removes the detection logic from the Requirement in favour of it living inside
the Gpg class & us calling it from there. It's a bit nicer & avoids us calling
Requirement code from outside of direct requirement handling & fulfillment.

GPG 1.x has stopped receiving new features, some of which we may well want to
take advantage of sooner or later in Homebrew. Upstream has also been attempting
to work out for a while how well used it still is which suggests it may "go away"
at some point in the future.

Debian is also in the process of migrating GnuPG 1.x to a `gpg1` executable
whilst GnuPG 2.1.x assumes the `gpg` executable. There's a detailed video
discussion of this from DebConf 2015 at:
http://meetings-archive.debian.net/pub/debian-meetings/2015/debconf15/GnuPG_in_Debian_report.webm

It's unsafe to assume every `gpg` executable is going to forever equal 1.x and
every `gpg2` executable is going to forever equal 2.x. MacGPG2 has been symlinking
2.x as a vanilla `gpg` for a while, for example, and we will be soon as well.

You'll still be able to plonk the `libexec/bin` path of `gpg` in your PATH to
access a vanilla `gpg` 1.x executable if you want to, but we're not going to
actively keep adding gpg1 support to formulae going forwards. There's really no
reason why 99.9% of projects should not or cannot use `gpg2` these days.

This uses detection methods to determine regardless of what the executable
is called we're always hitting a 2.0 GnuPG or nothing.

See Homebrew/legacy-homebrew#48449

which_all already runs some checks to see if the file is a file & is executable.
Our usage here inside `self.available?` is mostly a smoke test.

Closes #676.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>