Update minimum version of nokogiri in Gemfile also

What does this MR do?

It updates minimum version of nokogiri in Gemfile too (Gemfile.lock has it updated already). nokogiri 1.8.2 was working fine till gitlab 11.1.8 but does not work in gitlab 11.2.8

Fixes: ActionView::Template::Error (undefined method `add_class' for #<Nokogiri::XML::Element:0x0055dbff5252e8>

The failure was in /help page.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer