Minor optimizations

[syndbg]

• Updated Ruby and Rails versions
• Set up Rubocop (https://github.com/bbatsov/rubocop).
• Ran an Rubocop autocorrect to automagically fix issues (rubocop -a)
• Added indices to the most used table columns in queries
• Modified controller queries to raise ActiveRecord::NotFound exceptions. In RAILS_ENV=production this will show a 404 page

Addresses #12 in 4th bullet