Require plaintext signature method

.travis.yml

@@ -5,7 +5,7 @@ addons:
 
 language: ruby
 
-before_install: rvm rubygems master --force
+before_install: rvm rubygems v2.6.6 --force
 install: gem install --file
 script: "RUBYGEMS_GEMDEPS=- rake test"
 

lib/oauth.rb

@@ -7,6 +7,7 @@
 require 'oauth/core_ext'
 
 require 'oauth/client/helper'
+require 'oauth/signature/plaintext'
 require 'oauth/signature/hmac/sha1'
 require 'oauth/signature/rsa/sha1'
 require 'oauth/request_proxy/mock_request'

test/integration/consumer_test.rb

@@ -32,7 +32,6 @@ def test_that_signing_auth_headers_on_get_requests_works
     end
 
     def test_that_setting_signature_method_on_consumer_effects_signing
-      require 'oauth/signature/plaintext'
       request = Net::HTTP::Get.new(@request_uri.path)
       consumer = @consumer.dup
       consumer.options[:signature_method] = 'PLAINTEXT'
@@ -44,7 +43,6 @@ def test_that_setting_signature_method_on_consumer_effects_signing
     end
 
     def test_that_setting_signature_method_on_consumer_effects_signature_base_string
-      require 'oauth/signature/plaintext'
       request = Net::HTTP::Get.new(@request_uri.path)
       consumer = @consumer.dup
       consumer.options[:signature_method] = 'PLAINTEXT'
@@ -58,7 +56,7 @@ def test_that_setting_signature_method_on_consumer_effects_signature_base_string
 
     def test_that_plaintext_signature_works
       # Invalid test because server expects double-escaped signature
-      require 'oauth/signature/plaintext'
+
       # consumer = OAuth::Consumer.new("key", "secret",
       #   :site => "http://term.ie", :signature_method => 'PLAINTEXT')
       # access_token = OAuth::AccessToken.new(consumer, 'accesskey', 'accesssecret')

test/units/test_em_http_client.rb

@@ -27,7 +27,6 @@ def test_that_using_auth_headers_on_get_requests_works
   end
 
   def test_that_using_auth_headers_on_get_requests_works_with_plaintext
-    require 'oauth/signature/plaintext'
     c = OAuth::Consumer.new('consumer_key_86cad9', '5888bf0345e5d237',{
       :signature_method => 'PLAINTEXT'
     })

test/units/test_net_http_client.rb

@@ -25,7 +25,6 @@ def test_that_using_auth_headers_on_get_requests_works
   end
 
   def test_that_using_auth_headers_on_get_requests_works_with_plaintext
-    require 'oauth/signature/plaintext'
     c = OAuth::Consumer.new('consumer_key_86cad9', '5888bf0345e5d237',{
       :signature_method => 'PLAINTEXT'
     })

test/units/test_signature_plain_text.rb

@@ -1,5 +1,4 @@
 require File.expand_path('../../test_helper_units', __FILE__)
-require 'oauth/signature/plaintext'
 
 class TestSignaturePlaintext < Minitest::Test
   def test_that_plaintext_implements_plaintext