Allow 'brew install' on relative paths

Library/Homebrew/formula.rb

@@ -333,10 +333,10 @@ class Formula
       install_type = :from_url
     else
       # Check if this is a name or pathname
-      path = Pathname.new(name)
-      if path.absolute?
-        # For absolute paths, just require the path
+      if name.include? "/"
+        # For paths, just require the path
         require name
+        path = Pathname.new(name)
         name = path.stem
         install_type = :from_path
         target_file = path.to_s