Fix: Install webpacker before running the server

[vysogot]

There is a not defined in config/webpacker.yml error when running the command from the readme:
rails new searchapp --skip --skip-bundle --template https://raw.github.com/vysogot/elasticsearch-rails/master/elasticsearch-rails/lib/rails/templates/01-basic.rb

Here's the partial output from before the change:

run    rails server --port=3001 from "."
=> Booting Puma
=> Rails 6.0.0.beta3 application starting in development 
=> Run `rails server --help` for more startup options
RAILS_ENV=development environment is not defined in config/webpacker.yml, falling back to production environment
Exiting
Traceback (most recent call last):
	68: from bin/rails:4:in `<main>'
	67: from /Users/jgodawa/.rbenv/versions/2.6.0/lib/ruby/gems/2.6.0/gems/activesupport-6.0.0.beta3/lib/active_support/dependencies.rb:297:in `require'

The commit generates config/webpacker.yml file before it tries to run the server.

[vysogot]

There are bundler errors in the CI, not much to do with the commit

[estolfo]

thanks @vysogot !