Fix mismatch parallel dependency in Gemfile

The Gemfile specified version `~> 1.17.0` was mismatched with the
Gemfile.lock version `1.19.1`.

This seems have possibly occurred due to an issue with 2 different merge
requests changing this at the same time and being merged without
rebasing on each other:

- https://gitlab.com/gitlab-org/gitlab/merge_requests/20738
- https://gitlab.com/gitlab-org/gitlab/merge_requests/21886

Gemfile

@@ -389,7 +389,7 @@ group :development, :test do
 
   gem 'png_quantizator', '~> 0.2.1', require: false
 
-  gem 'parallel', '~> 1.17.0', require: false
+  gem 'parallel', '~> 1.19', require: false
 end
 
 # Gems required in omnibus-gitlab pipeline

Gemfile.lock

@@ -1287,7 +1287,7 @@ DEPENDENCIES
   omniauth_crowd (~> 2.2.0)
   omniauth_openid_connect (~> 0.3.3)
   org-ruby (~> 0.9.12)
-  parallel (~> 1.17.0)
+  parallel (~> 1.19)
   peek (~> 1.1)
   pg (~> 1.1)
   png_quantizator (~> 0.2.1)