Git ignoring vendor folder

I have added vendor folder to my .gitignore file, but still whenever I do git status it shows me that vendor folder is modified. Below are my steps that I followed in order to push my code to my repository:

1. Created .gitignore file with following content:

   .idea/*
   log/*
   tmp/*
   

2. Pushed .gitignore file to repository:

   git add .
   git commit -m "test"
   git push origin master
   

3. Created new project that contains vendor folder

4. Pushed project with vendor folder to repository:

   git add .
   git commit -m "test"
   git push origin master
   

5. Modified .gitignore file to ignore vendor folder:

   .idea/*
   log/*
   tmp/*
   vendor/bundle/*
   vendor/cache/*
   vendor/plugins/*
   

But wherever I do git status it shows:

modified:   vendor/bundle/ruby/1.9.1/bundler/gems/jquery-rails-f79e8f178

even if the vendor folder is in .gitignore file.