Rewrite plists when installing from a bottle

Library/Homebrew/extend/pathname.rb

@@ -98,7 +98,7 @@ class Pathname
 
   # we assume this pathname object is a file obviously
   def write content
-    #raise "Will not overwrite #{to_s}" if exist? and not ARGV.force?
+    raise "Will not overwrite #{to_s}" if exist? and not ARGV.force?
     dirname.mkpath
     File.open(self, 'w') {|f| f.write content }
   end

Library/Homebrew/formula_installer.rb

@@ -288,9 +288,9 @@ class FormulaInstaller
   end
 
   def install_plist
-    # Install a plist if one is defined
-    # Skip plist file exists check: https://github.com/mxcl/homebrew/issues/15849
     if f.startup_plist
+      # A plist may already exist if we are installing from a bottle
+      f.plist_path.unlink if f.plist_path.exist?
       f.plist_path.write f.startup_plist
       f.plist_path.chmod 0644
     end