Upgrade `json_schemer` to 2.3.0

What does this MR do and why?

This MR upgrades the json_schemer gem from 0.2.18 to 2.3.0, which is the latest version.

The version we've been on is a prerelease version that doesn't have all the features we need to migrate CI config validation to a JSON schema. It's also difficult to use because the documentation for it no longer exists.

Related to #467375

Benchmarking (values in milliseconds)

gem version	user	system	total	real
json_schemer-2.3.0	23.575	1.481	25.056	(24.896)
json_schemer-0.2.18 (currently in Gemfile)	4.575	0.688	5.263	(5.133)
voxpupuli/json-schema-4.3.1 (using #fully_validate)	41.373	5.279	46.652	(46.690)

Code used for benchmarking

# json_schemer

require 'benchmark'

ActiveRecord::Base.logger = nil

n = 5
schema_path = Rails.root.join('app/assets/javascripts/editor/schema/ci.json')
value = YAML.load_file('tmp-gitlab-ci.yml')

Benchmark.bm do |benchmark|
  benchmark.report("v0.2.18") do
    Gitlab::SafeRequestStore.ensure_request_store do
      n.times do
        validator = JSONSchemer.schema(Pathname.new(schema_path))

        validator.validate(value).to_a
      end
    end
  end
end

# json-schema

require 'benchmark'
require 'json-schema'

ActiveRecord::Base.logger = nil

n = 5
schema = Gitlab::Json.parse(File.read(Rails.root.join('app/assets/javascripts/editor/schema/ci.json')))
value = YAML.load_file('tmp-gitlab-ci.yml')

Benchmark.bm do |benchmark|
  benchmark.report("json-schema-4.3.1") do
    Gitlab::SafeRequestStore.ensure_request_store do
      n.times do
        JSON::Validator.fully_validate(schema, value)
      end
    end
  end
end