Test more recent versions of Ruby on Travis (#480)

* Test more recent versions of Ruby on Travis

Added:
- 2.4.9 (reached end-of-life but may temporarily help in incremental debugging)
- 2.5.8
- 2.6.6

Did not add 2.7.x yet because it introduces *a lot* of warnings.

Did not remove any past version yet because until decided otherwise,
they should be supported by the gem. (I would recommend a major version
bump when those versions get dropped.)

The idea is that developments from now on should be future-proof, and
changes should be tested on current versions of Ruby.

* Update WebMock to latest 2.x version

Updating to WebMock 2.x means that:

> require 'webmock' does not enable WebMock anymore. gem 'webmock' can
now be safely added to a Gemfile and no http client libs will be
modified when it's loaded. Call WebMock.enable! to enable WebMock.
>
> Please note that require 'webmock/rspec', require 'webmock/test_unit',
require 'webmock/minitest' and require 'webmock/cucumber' still do
enable WebMock.

Source: [WebMock's
CHANGELOG.md](https://github.com/bblimke/webmock/blob/master/CHANGELOG.md#200).

As rspec_api_documentation is very much tied to RSpec, I saw no problem
with replacing all instances of `require 'webmock'` with `require
'webmock/rspec'`.

* Update WebMock dependency to 3.2.0

3.2.0 introduced another breaking change that had to be addressed:

[Automatically disable WebMock after rspec suite](bblimke/webmock#731)

As the example app_spec.rb in features/oauth2_mac_client.feature is supposed to be a stand-alone spec file, it made sense, to me, to `require "webmock/rspec"`.
Does that make any sense?

* Bump WebMock dev dependency to 3.5.0

This introduces Ruby 2.6 support and should get the test suite to run on that version of Ruby.

* Try using WebMock 3.8.3

Let's see how this goes on Travis.

Author: davidstosik

.travis.yml

@@ -3,6 +3,9 @@ rvm:
   - 2.1.8
   - 2.2.4
   - 2.3.0
+  - 2.4.9
+  - 2.5.8
+  - 2.6.6
 gemfile:
   - Gemfile
 script:

features/oauth2_mac_client.feature

@@ -2,6 +2,7 @@ Feature: Use OAuth2 MAC client as a test client
   Background:
     Given a file named "app_spec.rb" with:
       """
+      require "webmock/rspec"
       require "rspec_api_documentation"
       require "rspec_api_documentation/dsl"
       require "rack/builder"

lib/rspec_api_documentation/dsl/resource.rb

@@ -30,7 +30,7 @@ def self.define_action(method)
 
       def callback(*args, &block)
         begin
-          require 'webmock'
+          require 'webmock/rspec'
         rescue LoadError
           raise "Callbacks require webmock to be installed"
         end

lib/rspec_api_documentation/oauth2_mac_client.rb

@@ -4,7 +4,7 @@
   # ActiveSupport::SecureRandom not provided in activesupport >= 3.2
 end
 begin
-  require "webmock"
+  require "webmock/rspec"
 rescue LoadError
   raise "Webmock needs to be installed before using the OAuth2MACClient"
 end

rspec_api_documentation.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rake", "~> 10.5.0"
   s.add_development_dependency "rack-test", "~> 0.6.3"
   s.add_development_dependency "rack-oauth2", "~> 1.2.2"
-  s.add_development_dependency "webmock", "~> 1.22.6"
+  s.add_development_dependency "webmock", "~> 3.8.3"
   s.add_development_dependency "rspec-its", "~> 1.2.0"
   s.add_development_dependency "faraday", "~> 0.9.2"
   s.add_development_dependency "nokogiri", "~> 1.8.4"

spec/http_test_client_spec.rb

@@ -3,7 +3,7 @@
 require 'capybara'
 require 'capybara/server'
 require 'sinatra/base'
-require 'webmock'
+require 'webmock/rspec'
 require 'support/stub_app'
 
 describe RspecApiDocumentation::HttpTestClient do