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Fixes #1223.

This is needed since some formulae now set

  ENV["GLIDE_HOME"] = HOMEBREW_CACHE/"glide_home/#{name}"

`Homebrew/>1.0.0 (no git repository) (Macintosh; Intel macOS 10.7.5)...)
reads pretty weirdly in a user agent and I've had complaints that `>`
may be an invalid character in some cases.

* Fix IntegrationCommandTestCask

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
```