Upgrade Webmock to 3.5.1

Webmock 3.5.0 brings Ruby 2.6 support.

Gemfile

@@ -380,7 +380,7 @@ group :test do
   gem 'shoulda-matchers', '~> 3.1.2', require: false
   gem 'email_spec', '~> 2.2.0'
   gem 'json-schema', '~> 2.8.0'
-  gem 'webmock', '~> 2.3.2'
+  gem 'webmock', '~> 3.5.1'
   gem 'rails-controller-testing'
   gem 'sham_rack', '~> 1.3.6'
   gem 'concurrent-ruby', '~> 1.1'

Gemfile.lock

@@ -357,7 +357,7 @@ GEM
       thor
       tilt
     hangouts-chat (0.0.5)
-    hashdiff (0.3.4)
+    hashdiff (0.3.8)
     hashie (3.5.7)
     hashie-forbidden_attributes (0.1.1)
       hashie (>= 3.0)
@@ -925,7 +925,7 @@ GEM
     vmstat (2.3.0)
     warden (1.2.7)
       rack (>= 1.0)
-    webmock (2.3.2)
+    webmock (3.5.1)
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff
@@ -1175,7 +1175,7 @@ DEPENDENCIES
   version_sorter (~> 2.1.0)
   virtus (~> 1.0.1)
   vmstat (~> 2.3.0)
-  webmock (~> 2.3.2)
+  webmock (~> 3.5.1)
   webpack-rails (~> 0.9.10)
   wikicloth (= 0.8.1)