Fix nokogiri dependency: demand ~> 1.5.10 instead of < 1.6.0

The previous nokogiri dependency (`< 1.6.0`) gave issues in our projects in combination with capybara, possibly due to a dependency resolving bug in Bundler. This has been mentioned in https://github.com/aws/aws-sdk-ruby/issues/273, https://github.com/bundler/bundler/issues/2122, and https://github.com/bundler/bundler/issues/2593.

This PR demands version `~> 1.5.10`, which is effectively the same as demanding anything `< 1.6.0`, but after applying this PR our projects can `bundle update` normally with capybara and aws-sdk in the `Gemfile`.