CI: Change and test earliest supported Ruby version (1.9.2 -> 2.2.0) (#498)

* gemspec: Change required Ruby version from 1.9.2 to 2.2.0

There are some incompatibilities between Split's dependencies and
earlier versions of Ruby (see
#494 (comment)).
2.2.0 is a relatively old version of Ruby that can run Split with
minimal code changes.

* Fix Ruby syntax incompatibility with version < 2.3

`Hash#dig` and `&.` are available only in 2.3+, but Split is supposed to
be compatible with Ruby 2.2.2+.

* CI: Add Rails 5.1 Gemfile to build matrix

Author: patbl

.travis.yml

@@ -1,10 +1,20 @@
 language: ruby
 rvm:
+  - 2.2.0
+  - 2.2.2
   - 2.4.1
 
 gemfile:
   - gemfiles/4.2.gemfile
   - gemfiles/5.0.gemfile
+  - gemfiles/5.1.gemfile
+
+matrix:
+  exclude:
+  - rvm: 2.2.0
+    gemfile: gemfiles/5.0.gemfile
+  - rvm: 2.2.0
+    gemfile: gemfiles/5.1.gemfile
 
 before_install:
   - gem install bundler

lib/split/helper.rb

@@ -10,7 +10,7 @@ def ab_test(metric_descriptor, control = nil, *alternatives)
         experiment = ExperimentCatalog.find_or_initialize(metric_descriptor, control, *alternatives)
         alternative = if Split.configuration.enabled
           experiment.save
-          raise(Split::InvalidExperimentsFormatError) unless Split::configuration.experiments&.dig(experiment.name.to_sym,:combined_experiments).nil?
+          raise(Split::InvalidExperimentsFormatError) unless (Split.configuration.experiments || {}).fetch(experiment.name.to_sym, {})[:combined_experiments].nil?
           trial = Trial.new(:user => ab_user, :experiment => experiment,
               :override => override_alternative(experiment.name), :exclude => exclude_visitor?,
               :disabled => split_generically_disabled?)

split.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
    "mailing_list_uri" => "https://groups.google.com/d/forum/split-ruby"
  }
 
-  s.required_ruby_version = '>= 1.9.2'
+  s.required_ruby_version = '>= 2.2.0'
 
   s.rubyforge_project = "split"