Update dependency nokogiri to >= 1.10.0 to support ruby 2.6

[Simran-B]

I want to report a bug:

Jekyll works fine with Ruby v2.6, but the jekyll-algolia gem depends on a version of nokogiri which supports Ruby < 2.6 only. They added support for Ruby 2.6 in a more recent version:
sparklemotion/nokogiri@d8addcd

What is the current behavior?

Installing the gems with bundler fails if your Ruby version is >= 2.6.0:

>bundle install
Fetching gem metadata from https://rubygems.org/...........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "ruby ":
  In Gemfile:
    ruby

    jekyll-algolia (~> 1.0) was resolved to 1.4.11, which depends on
      nokogiri (~> 1.6) was resolved to 1.8.5, which depends on
        ruby  (< 2.6) x64-mingw32

Could not find gem 'ruby  (< 2.6)', which is required by gem 'nokogiri (~>
1.6)', in any of the relevant sources:
  the local ruby installation

What is your expected behavior?

jekyll-algolia should use a more recent version of nokogiri to support Ruby 2.6 (out for over half a year).

Git repository to reproduce the issue:

https://github.com/arangodb/docs

Ruby version used:

ruby 2.6.1p33 (2019-01-30 revision 66950) [x64-mingw32]

Jekyll version used:

jekyll 3.8.5