Remove `:bsexec` parameter from `SystemCommand`.

formula, ARGV: don't output duplicate formulae.

keg: fall back for dependencies of buggy tabs

keg: don't return nil dependencies

keg: fix fallback dependencies with multiple kegs

If you specify a formula more than once or it exists in the Cellar with
an alias name and the main name (e.g. `qt` and `qt5`) you can see the
same formula showing up more than once. Instead, resolve these output
lists of formulae such that they are unique based on their `name`. This
doesn't use `full_name` as it's `name` that's use for the `Cellar`.

This is a proper fix to the problem addressed by #1510.

The problem arises when f_kegs is nil, which can happen if the name and
tap used to install a keg don't match the name and tap currently
associated with its formula (i.e. if it's been renamed or moved).

This reverts commit d7e72899.

See https://github.com/Homebrew/brew/pull/1752#discussion_r94257147

formula_versions: handle uncommitted formulae.

ENV: deprecate old methods.

audit: improve reliability of homepage audit.

See https://github.com/Homebrew/brew/pull/1750#discussion_r94243825 for
discussion.

Removes Tab#reliable_runtime_dependencies? in favour of returning nil
from Tab#runtime_dependencies if the list is unreliable.

Because Homebrew 1.1.6 hasn't been tagged yet, tabs created in tests
aren't created with a homebrew_version that marks the
runtime_dependencies in the Tab as reliable, so there are some tests
that fail. To work around this, I've had to add a line to some tests
that explicitly overrides the homebrew_version in the Tab. This is
really ugly though, so they should be removed as soon as possible after
1.1.6 is released.

These should have been deprecated long ago but we didn’t really have
the necessary framework to do so.

This will use Curl’s default user agent to reduce homepage errors and
provides a function that can be used for other audits to perform
similar tests on URLs.

uninstall: a couple of dependency error/warning message improvements

Otherwise an uncommitted formula can trigger an invalid `brew audit`
result.

Fixes #1731.

Fixes #1524.

This method isn't used any more

It didn't occur to me that this would work, but it does! Magic.

The example command given in the dependency errors/warnings previously
comma-seperated the kegs, which was incorrect syntax. This fixes that.

Previously, this only included a subset of all the kegs the user was
trying to uninstall. With this change, all named formulae/kegs will be
included in the example command.

Deprecate `brew cask update`.

formula_installer: optional deps version check.

Fixes #1554.

cc: always filter flags on deps.

Require `HOMEBREW_CHECK_RECURSIVE_VERSION_DEPENDENCIES` to be specified
(which will be by `brew test-bot`) to avoid this being inflicted on
end-users unnecessarily.

This has been tested by `brew test-bot` since March and there’s been no
issues so let’s turn it on by default.

tap_migrations: support renaming to/from casks.