I added a new `FormulaUnreadableError` subclass of
`FormulaUnavailableError` so existing `rescue`s of
`FormulaUnavailableError` handle this as well.

The new subclass will output the name of the formula with the error
(because this isn't always obvious from the original exception message)
followed by the original error message.

Fixes #1927.

Try "diskutil eject" twice before "diskutil unmount force".

`brew cleanup` should cleanup formulae which have the older
version scheme.

As requested in https://github.com/Homebrew/homebrew-core/issues/9316.

If we make a PR in which we're building many formulae (e.g.
https://github.com/Homebrew/homebrew-core/pull/9249) then we may want
to pull those that built and publish their bottles and discard those
that did not. Instead of saying bottles will be published when they
won't and then blowing up just avoid publishing them and print a warning
instead.

Otherwise it won't overwrite unmerged changes.

Try first with an (exact, actual) browser user agent from Safari and
then try again with the default Homebrew `curl` user agent.

This directory indicates that configuration files have been installed
into the wrong location.

This is warned by the newer MiniTest.

This avoids a `rake` dependency in the Gemfile and an unnecessary shell
out.

We’re upgrading SimpleCov to 0.13.0 so let’s update the rest of our
gems while we’re at it.

This is cleaner, easier to understand how the arguments are split and
fixes #1799.