Update dependency nokogiri to >= 1.10.0 to support ruby 2.6

## 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:
https://github.com/sparklemotion/nokogiri/commit/d8addcd3691b4995694a1c7f33fa18e3c740a829

### 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`

Start with the jekyll-algolia dependency declaration and reproduce the reported bundle install failure using Ruby 2.6. Confirm that the dependency resolution permits nokogiri 1.10.0 or newer and that installation succeeds for the reported environment.

Written by the indexing model from the issue text.