Restrict denvig-alpha completions to zsh with no shell suffix

In 0.7.0, 'denvig zsh completions' rejects extra arguments. The default
generate_completions_from_executable iterates over bash/zsh/fish and
appends the shell name, producing 'denvig zsh completions bash' which
errors. Pin shells to zsh only and use shell_parameter_format :none so
the command runs as 'denvig zsh completions'.

Author: marcqualie

Formula/denvig-alpha.rb

@@ -20,7 +20,7 @@ def install
     system "npm", "install", *std_npm_args
     bin.install_symlink libexec.glob("bin/*")
 
-    generate_completions_from_executable(bin/"denvig", "zsh", "completions")
+    generate_completions_from_executable(bin/"denvig", "zsh", "completions", shells: [:zsh], shell_parameter_format: :none)
   end
 
   test do