Simplify installation of brew caveats command

[superatomic]

This pull request simplifies the installation process of the brew caveats command which this repository provides.

As explained in the documentation for external commands, external commands can be added to brew simply by tapping a repository with a cmd/ directory, without needing to brew install anything. An example of this is my homebrew-bundle-extensions project, where the repository only needs to be tapped for the commands to be added.

This PR makes cmd/brew-caveats.rb executable (by running chmod +x cmd/brew-caveats.rb), which is all that is needed simplify the installation command to be brew tap rafaelgarrido/caveats, with no need to run brew install brew-caveats.

This PR also removes the Formula/brew-caveats.rb installation formula, as it is no longer required to install brew caveats. This change is backwards compatible because everyone who installed brew caveats this way also tapped rafaelgarrido/caveats first.