virtualenvs remember the prefix of the python that created them and do
magic to load packages from them; help them remember a more durable
path.

`virtualenv_install_with_resources` will now attempt to guess the
desired Python based on the active build options and based on the
dependencies and requirements. When the situation is ambiguous (e.g.,
`depends_on :python3` and `build.with? "python"` is true) raise
`FormulaAmbiguousPythonError` unless `:using => "python"` or
`:using => "python3"` has been passed to resolve the ambiguity.

In most cases, this will allow

```
    virtualenv_create(libexec, "python3")
    virtualenv_install_with_resources
```

to be changed to just

```
    virtualenv_install_with_resources
```

These are the ones that are auto-installed without tapping and it makes
sense to make them more prominent in our documentation as we've
offloaded various functionality over the years to each of them.

Update some messaging based on recent user confusion and the
`/usr/local/Homebrew` and Ruby 2 migrations.

Fix updating the `stable` branch used for tagged updates and ensure
that the core tap isn’t retapped (the slowest part by far of this test).

`brew list --verbose <formulae>` lists only those kegs that
are installed and exits with 0 if all <formulae> installed.
If one or more formulae from <formulae> are not installed, the
command lists only installed versions and exits with 1.

`brew --pinned formula` exits with 1 if formula is not pinned.

Fixes #1172

This is no longer relevant after moving to taps/tags and if ever
re-added it should probably target homebrew/core.

Fixes #1194.