Update Appveyor configs:

- Update build image to Visual Studio 2019 (Windows Server 2019).
- Use x64 builds
- Disable doc generation to speed up gem update/install.

appveyor.yml

@@ -2,30 +2,30 @@ build: off
 cache:
   - vendor/bundle
 image:
-  - Visual Studio 2017
+  - Visual Studio 2019
 environment:
   matrix:
-    - RUBY_VERSION: Ruby24
+    - RUBY_VERSION: Ruby24-x64
       RUBY_BIN: ruby
-      RAKE_TASK: spec
+      RAKE_TASK: rubocop
       SCRIPT_CONTEXT: bundle exec
-    - RUBY_VERSION: Ruby24
+    - RUBY_VERSION: Ruby24-x64
       RUBY_BIN: ruby
-      RAKE_TASK: rubocop
+      RAKE_TASK: spec
       SCRIPT_CONTEXT: bundle exec
-    - RUBY_VERSION: Ruby25
+    - RUBY_VERSION: Ruby25-x64
       RUBY_BIN: ruby
       RAKE_TASK: spec
       SCRIPT_CONTEXT: bundle exec
-    - RUBY_VERSION: Ruby26
+    - RUBY_VERSION: Ruby26-x64
       RUBY_BIN: ruby
       RAKE_TASK: spec
       SCRIPT_CONTEXT: bundle exec
     - RUBY_VERSION: jruby-9.2.11.1
       RUBY_BIN: jruby
       RAKE_TASK: spec
       SCRIPT_CONTEXT: jruby -G -S
-    - RUBY_VERSION: Ruby26
+    - RUBY_VERSION: Ruby26-x64
       RUBY_BIN: ruby
       RAKE_TASK: spec
       SCRIPT_CONTEXT: bundle exec
@@ -34,8 +34,8 @@ install:
   - ps: if ($env:RUBY_BIN -eq 'jruby') { support\install_jruby.ps1 }
   - ps: support\install_msedge.ps1
   - set PATH=C:\%RUBY_VERSION%\bin;%PATH%
-  - '%RUBY_BIN% -S gem update --system'
-  - '%RUBY_BIN% -S gem install bundler'
+  - '%RUBY_BIN% -S gem update --system -N'
+  - '%RUBY_BIN% -S gem install bundler -N'
   - '%RUBY_BIN% -S bundle install'
 before_test:
   - '%RUBY_BIN% -v'