* Upgrate to rails 6.0.0

* Simplify Rails.gitignore

https://weblog.rubyonrails.org/2019/3/13/Rails-4-2-5-1-5-1-6-2-have-been-released/
Rails `5.2.2.1` requires `tmp` directory committed to git repository for CI to run successfully.

Rails 5 uses npm to handle javascript dependancies, node_modules should not be checked in to git.

`**.` is invalid

Rails no longer generates this file with sensitive secrets (default now is to do `secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>` in production).

Many twelve-factor apps use environment variables rather than the built in Rails secrets mechanism to store secrets.
The [dotenv](https://github.com/bkeepers/dotenv) gem is widely used for this purpose and allows environment variables to be loaded from the `.env` file. However, this file should not be committed to source control.

This PR just adds the `.env` file to `.gitignore` so that these secrets aren't accidentally committed. In some rare instances, it may be required that this file is committed to source control so I've added it to the list of files that can be removed if the developer is ok with secrets being committed.

.byebug_history included in default Rails5 setup (https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/gitignore)

I know this is petty, but imagine thousand upon thousand of corrective commits saved by fixing the typo right into the heart of the file.

Couldn't find any references to .powconfig  being used in application directory

bower-rails integrates bower.io into Rails, it allows you to have everything in a Bowerfile and install all vendor assets easily.

I added secret_token.rb file to the gitignore as there are public repos sharing it.

.project files come from Aptana Studio, a Rails IDE.

The possible files could be database.yml or sold.yml

Unless full paths are used .gitignore, all matching files and folders will be ignored, however deep they are in the repo.

For example, ignoring log/* doesn't just ignore all log files, but also files such as lib/project/log/awesome_logger.rb.  It's a mistake I've made a couple of times but never seemed to have learned from.