Add "build_dependencies" key to JSON output (#340)

This gives the JSON output the same type of information as
`recommended_dependencies` or `optional_dependencies`, but for those
marked `:build` in the formula.

Library/Homebrew/formula.rb

@@ -1256,6 +1256,7 @@ class Formula
       "dependencies" => deps.map(&:name).uniq,
       "recommended_dependencies" => deps.select(&:recommended?).map(&:name).uniq,
       "optional_dependencies" => deps.select(&:optional?).map(&:name).uniq,
+      "build_dependencies" => deps.select(&:build?).map(&:name).uniq,
       "conflicts_with" => conflicts.map(&:name),
       "caveats" => caveats
     }