add rubocop to travis config

Gemfile

@@ -58,9 +58,6 @@ gem 'tabler-rubygem'
 # Use Capistrano for deployment
 # gem 'capistrano-rails', group: :development
 
-# Ruby linting.
-gem 'rubocop', require: false
-
 group :production do
   # Use a postgres database in production.
   gem 'pg', '~> 0.18'
@@ -82,6 +79,9 @@ group :test do
   gem 'shoulda-matchers', '~> 3.1'
   gem 'faker'
   gem "factory_bot_rails"
+
+  # Ruby linting.
+  gem 'rubocop'
 end
 
 group :development do