Fix requiring the rubyzip Gem

In commit 6fa5fd85 the `require: false`
was removed to ensure the Gem was loaded at run time. Unfortunately, the
`require` necessary for the rubyzip Gem is "zip" and not "rubyzip". As a
result, Bundler would not require the Gem. This meant that we would
still run into constant errors when referring to `Zip::File`.

Gemfile

@@ -57,7 +57,7 @@ gem 'u2f', '~> 0.2.1'
 
 # GitLab Pages
 gem 'validates_hostname', '~> 1.0.6'
-gem 'rubyzip', '~> 1.2.2'
+gem 'rubyzip', '~> 1.2.2', require: 'zip'
 
 # Browser detection
 gem 'browser', '~> 2.5'