Use bundler.vim

Includes features such as:

* `:Bundle`, which wraps `bundle`.
* An internalized version of `bundle open`: `:Bopen` (and `:Bsplit`,
  `:Btabedit`, etc.).
* `'path'` and `'tags'` are automatically altered to include all gems
  from your bundle.  (Generate those tags with
  [gem-ctags](https://github.com/tpope/gem-ctags)!)
* Highlight Bundler keywords in `Gemfile`.
* Support for `gf` in `Gemfile.lock`, plus syntax highlighting that
  distinguishes between installed and missing gems.