Remove deprecated Ruby. Upgrade bundler. Fix failing tests

.travis.yml

@@ -1,10 +1,13 @@
+sudo: true
+service: docker
 language: ruby
 cache:
 - bundler
+before_install:
+  - sudo iptables -L DOCKER || ( echo "DOCKER iptables chain missing" ; sudo iptables -N DOCKER )
 install:
 - bundle install
 rvm:
-- 2.3.0
 - 2.4.1
 notifications:
   email:
@@ -26,7 +29,6 @@ deploy:
   on:
     tags: true
     all_branches: true
-    rvm: 2.3.0
     rvm: 2.4.1
     repo: sensu-plugins/sensu-plugins-ssl
 - provider: script

CHANGELOG.md

@@ -4,6 +4,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This CHANGELOG follows the format listed [here](https://github.com/sensu-plugins/community/blob/master/HOW_WE_CHANGELOG.md).
 
 ## [Unreleased]
+- Remove ruby-2.3.0. Upgrade bundler. Fix failing tests (@phumpal).
+
 ### Breaking Changes
 - Bump `sensu-plugin` dependency from `~> 1.2` to `~> 4.0` you can read the changelog entries for [4.0](https://github.com/sensu-plugins/sensu-plugin/blob/master/CHANGELOG.md#400---2018-02-17), [3.0](https://github.com/sensu-plugins/sensu-plugin/blob/master/CHANGELOG.md#300---2018-12-04), and [2.0](https://github.com/sensu-plugins/sensu-plugin/blob/master/CHANGELOG.md#v200---2017-03-29)
 

sensu-plugins-ssl.gemspec

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 lib = File.expand_path('../lib', __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 
@@ -25,7 +27,7 @@ Gem::Specification.new do |s|
   s.platform               = Gem::Platform::RUBY
   s.post_install_message   = 'You can use the embedded Ruby by setting EMBEDDED_RUBY=true in /etc/default/sensu'
   s.require_paths          = ['lib']
-  s.required_ruby_version  = '>= 2.1.0'
+  s.required_ruby_version  = '>= 2.4.0'
 
   s.summary                = 'Sensu plugins for SSL'
   s.test_files             = s.files.grep(%r{^(test|spec|features)/})