`rack` is a commonly used alias for `formula.prefix.parent`---so common that it
gets defined and used quite a bit. This patch makes `rack` an official method
of the `Formula` class.
Consequence: you can no longer install when something is already installed, you must upgrade it. This doesn't apply if the formula in question was unlinked. You can still --force installs though.
Rationale: the old way of installing over the top would leave symlinks to multiple versions in /usr/local if the old version had a file the newer version didn't. The new upgrade command handles everything properly.
