Upgrade paper_trail gem to most recent version

The issue paper-trail-gem/paper_trail#738 that
was locking paper_trail to the version we were currently using was
solved and its available since 6.X.

For all purposes, this also takes the opportunity to upgrade the
version to the lost recent one.

I've been through all the deprecation notes in the gem and it doesn't
seems there's anything on our code impacted by it.

NOTE: This also solves the deprecation notice that was appearing when
running RSpec:

```
DEPRECATION WARNING: before_filter is deprecated and will be removed in
Rails 5.1. Use before_action instead. (called from <top (required)> at
config/application.rb:8)
```

Gemfile

@@ -34,7 +34,7 @@ gem 'logstasher',             '0.6.2'
 gem 'logstuff',               '0.0.2'
 gem 'newrelic_rpm',           '~> 5.1.0.344'
 gem 'paperclip',              '~> 5.3.0'
-gem 'paper_trail',            '4.0.2' # version locked, /~https://github.com/airblade/paper_trail/issues/738
+gem 'paper_trail',            '~> 9.0.2'
 gem 'pdf-forms'
 gem 'pg',                     '~> 0.18.2'
 gem 'rails',                  '>= 5.0.0', '< 5.1'

Gemfile.lock

@@ -307,7 +307,7 @@ GEM
     http-cookie (1.0.3)
       domain_name (~> 0.5)
     http_parser.rb (0.6.0)
-    i18n (1.0.0)
+    i18n (1.0.1)
       concurrent-ruby (~> 1.0)
     i18n-tasks (0.9.20)
       activesupport (>= 4.0.2)
@@ -401,9 +401,8 @@ GEM
     notifications-ruby-client (2.6.0)
       jwt (>= 1.5, < 3)
     orm_adapter (0.5.0)
-    paper_trail (4.0.2)
-      activerecord (>= 3.0, < 6.0)
-      activesupport (>= 3.0, < 6.0)
+    paper_trail (9.0.2)
+      activerecord (>= 4.2, < 5.3)
       request_store (~> 1.1)
     paperclip (5.3.0)
       activemodel (>= 4.2.0)
@@ -439,7 +438,7 @@ GEM
     pry-rails (0.3.6)
       pry (>= 0.10.4)
     puma (3.11.2)
-    rack (2.0.4)
+    rack (2.0.5)
     rack-accept (0.4.5)
       rack (>= 0.4)
     rack-contrib (2.0.1)
@@ -492,7 +491,7 @@ GEM
     rdoc (4.3.0)
     redis (3.3.5)
     remotipart (1.3.1)
-    request_store (1.4.0)
+    request_store (1.4.1)
       rack (>= 1.4)
     responders (2.4.0)
       actionpack (>= 4.2.0, < 5.3)
@@ -722,7 +721,7 @@ DEPENDENCIES
   net-ssh
   newrelic_rpm (~> 5.1.0.344)
   nokogiri (~> 1.8)
-  paper_trail (= 4.0.2)
+  paper_trail (~> 9.0.2)
   paperclip (~> 5.3.0)
   parallel_tests
   pdf-forms