Move version information to independent file

Reduce the amount of code loaded to determine the version of
RbReadline to be used for packaging.

Do this in preparation to move gem specification to it's own file.
No longer build zip packages

Remove the generation of zip packages and instead rely on GitHub
'Releases' feature to provide them.
Ease use of gem with Bundler

Moving gemspec out of Rakefile and to a dedicated gemspec file
helps Bundler and other developers to use this project directly
from the repository, special for testing changes.
Reorder some gem metadata

Move authors and license information close to description and
version metadata.
Update test and explicitly define test dependency

Be explicit by making minitest a known development dependency.

Adjust test cases and relocate support modules to have a cleaner
test directory structure.
Add convenience Gemfile

This Gemfile serves only for those using Bundler workflow to
install the dependencies defined in the gem specification.