Update and relax rubocop dependency.

0.82 is actually the latest release, but it drops support for MRI 2.3, which we're still testing against. This way we're allowing for more modern Rubocop versions where possible, but enforcing the latest release for 2.3 support.

combustion.gemspec

@@ -27,6 +27,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency "pg"
   s.add_development_dependency "rails"
   s.add_development_dependency "rspec"
-  s.add_development_dependency "rubocop", "~> 0.79.0"
+  s.add_development_dependency "rubocop", "~> 0.81"
   s.add_development_dependency "sqlite3"
 end