add bundler setup requirement

[JensTimmerman]

this is needed to make whatweb work after following the installation instructions.

The current installation instructions for manual install tel me to

git clone repo
cd WhatWeb
bundle install
-> Your user account isn't allowed to install to the system RubyGems.
You can cancel this installation and run:

  bundle config set --local path 'vendor/bundle'
  bundle install

So I run
bundle config set --local path 'vendor/bundle'
bundle install
-> Bundle complete! 11 Gemfile dependencies, 28 gems now installed.
Bundled gems are installed into ./vendor/bundle

./whatweb
WhatWeb is not installed and is missing dependencies.
The following gems are missing:

• addressable

To install run the following command from the WhatWeb folder:
'bundle install'

running bundle exec whatweb does work
ruby -rbundler/setup whatweb also works

And so this patch also works.