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'`.

Author: davidstosik

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"
+  s.add_development_dependency "webmock", "2.0.0"
   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