powershell 6.0.0-alpha.12: add dependency on openssl and a note with …

[umireon]

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

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According to official docs, Homebrew's openssl and curl --with-openssl are the prerequisites for PowerShell.

openssl is specified with depends_on because it is required for some commands such as Invoke-WebRequest (a PowerShell's parallel to wget and curl).

In contrast, curl is not specified but noted in caveats for the some reasons.
The OpenSSL-backed libcurl is difficult to depend on because it requires a non-standard option (--with-openssl).
Furthermore, the parts depending on it are hard to invoke from PowerShell (but #C).
For more infomation, see PowerShell/PowerShell#2211