Add configuration for Ruby versions for development

[jtorreggiani]

Running the server locally we ran into an issue with a missing dependency for web brick which causes the development server to fail. This is due to a discrepancy in ruby versions. We should add a .ruby-version file to the root of the repository to specific the ruby version. This can be use by Ruby Version Manager and rbenv to used the correct ruby version.

This also means that the repository is likely behind on Ruby updates that should be handled as a separate update.