Add rubocop

[houndci](https://houndci.com) was try to add it because I thought set of tools that can be found in the local environment is want I 'm handy when you pull request

.rubocop.yml

+inherit_from: .rubocop_todo.yml
+AllCops:
+  Exclude:
+    - 'vendor/**/*'
+    - 'gemfiles/vendor/**/*'

Gemfile

@@ -17,4 +17,5 @@ gem 'pry'
 group :development, :test do
   gem 'mime-types', '~> 1.16'
   gem 'builder'
+  gem 'rubocop', require: false
 end

gemfiles/3.2.gemfile

@@ -14,6 +14,7 @@ gem "paperclip", :path => "../"
 group :development, :test do
   gem "mime-types", "~> 1.16"
   gem "builder"
+  gem "rubocop", :require => false
 end
 
 gemspec :path => "../"

gemfiles/4.0.gemfile

@@ -14,6 +14,7 @@ gem "paperclip", :path => "../"
 group :development, :test do
   gem "mime-types", "~> 1.16"
   gem "builder"
+  gem "rubocop", :require => false
 end
 
 gemspec :path => "../"

gemfiles/4.1.gemfile

@@ -14,6 +14,7 @@ gem "paperclip", :path => "../"
 group :development, :test do
   gem "mime-types", "~> 1.16"
   gem "builder"
+  gem "rubocop", :require => false
 end
 
 gemspec :path => "../"

gemfiles/4.2.gemfile

@@ -14,6 +14,7 @@ gem "paperclip", :path => "../"
 group :development, :test do
   gem "mime-types", "~> 1.16"
   gem "builder"
+  gem "rubocop", :require => false
 end
 
 gemspec :path => "../"