audit: formally ban go get usage

pull: fix status code check in check_bintray_mirror

The status code of the last redirect should be 2xx to be deemed successful.

Update `plist` to 3.3.0.

There's been an informal ban for a while but let's
be punchier because this crops up still.

audit: tweak dependency option audit.

Make sure that `if build.with?` isn't caught.

docs: tweak CSS.

Import new brew.sh CSS changes to improve the appearance of tables and
links.

tap-new: bump Xcode to 8.3.

brew.sh: tweak macOS user agent again.

Update this to the current latest Travis CI supports (which we're using)

Safari uses `Mac OS X` not `OS X`. To avoid contaminating other parts of
Homebrew use a separate variable for just this part, too.

brew.sh: use OS X in user agent rather than macOS.

This matches what Safari does on macOS 10.12 and is generally what
tools like Google Analytics expect the format to be.

audit: fix audit of new formulae.

When auditing new formulae without `--new-formula` the
`audit_revision_and_version_scheme` method fails ungracefully. Instead,
set some better defaults so fewer checks are needed.

Fixes #2551.

software_spec: process macOS versions on Linux.

This module doesn't actually have any behaviour that's problematic at
runtime on Linux so we may as well use it to properly sort macOS
versions there.

formula_installer: improve install/upgrade message

If you `brew install` a formula that's already installed you get:
  Warning: ripgrep-0.5.1 already installed

If you `brew install` an outdated formula that's installed you get:
  Error: ripgrep-0.5.1 already installed. To install this version, first
  `brew unlink ripgrep`

Instead, suggest that the user should `brew upgrade` in this case. If
the formula isn't outdated use the previous message.

audit: fix use of search_tap method.

This was removed in #2540 but this call site was note updated to use
the `search_taps` method instead.

Add remote search to `brew cask search`.

xcode: update Sierra CLT version to 802.0.42

Allow forcing Homebrew GitHub organisation.