Implement `brew cask upgrade`

bin/brew: don't set empty, unfiltered variables.

Otherwise we unconditionally set e.g. `CI`, `TRAVIS`.

Enable environment filtering for developers.

More environment filtering fixes

- Make `brew pull` pass through Git environment variables
- Whitelist all `TRAVIS_` variables.

If folks have set `HOMEBREW_DEVELOPER` or run a `dev-cmd` then enable
environment filtering by default for them.

Allow optionally using Homebrew's curl

Cleaner: ELF files are executable on Linux

- Include tests in context
- replace 'route' with 'path'
- more assorted fixes

If `HOMEBREW_FORCE_BREWED_CURL` is set then allow it to be used instead
of the system `curl`.

Add test/support/fixtures/elf/
and test/support/fixtures/tarballs/testball-0.1-linux.tbz

Add CVSRequirement and SubversionRequirement

formula_installer: force fewer default formulae

Use these two requirements in DependencyCollector.
Remove the unused function MacOS::Xcode.provides_cvs?

Don't force a default formulae install if a requirement is satisfied if
we're building a bottle and it's a build-time dependency.

setup-analytics: Handle missing /proc/.../uuid

lines_cop: Add condition to prevent false positives

- Simplify outdated Casks checks
- Make use of RSpec's let(:) and .and syntax

Fix SystemCommand escaping.

Change the generic bottle tag to processor-system

Installing casks is supported only on macOS

Cask doctor: add macOS & Java versions

bump-formula-pr: fix for when hub isn't configured

improve 'brew cask _stanza' by checking for known stanzas

ENV: don't set MACOSX_DEPLOYMENT_TARGET

Use patch found in the PATH