(maint) Add Ruby 2.7 to AppVeyor test matrix

AppVeyor only provides Ruby 2.7 with VS2019 images, and we are currently
using VS2015. For some reason, runs with VS2019 take longer than 20
minutes, so update the build matrix to only run Ruby 2.7 with VS2019.

In addition, VS2019 has the RUBYOPT variable set to `-Eutf-8` which
forces Ruby to use UTF-8 encoding, causing some spec tests to fail due
to them expecting different encodings. Unset the variable.

appveyor.yml

@@ -1,17 +1,26 @@
 version: 4.1.0.{build}
+image:
+  - Visual Studio 2015
+  - Visual Studio 2019
 clone_depth: 10
 install:
 build: off
 matrix:
   fast_finish: true
   exclude:
+    - platform: Ruby27-x64
+      image: Visual Studio 2015
+    - platform: Ruby26-x64
+      image: Visual Studio 2019
     - platform: Ruby25-x64
-      LOCALE: ja-JP
-    - platform: Ruby24-x64
+      image: Visual Studio 2019
+    - platform: Ruby25-x64
+      image: Visual Studio 2015
       LOCALE: ja-JP
 
 # Ruby versions under test
 platform:
+  - Ruby27-x64
   - Ruby26-x64
   - Ruby25-x64
 
@@ -51,6 +60,9 @@ test_script:
       $Env:PATH = "C:\${Env:PLATFORM}\bin;${Env:PATH}"
       $Env:LOG_SPEC_ORDER = 'true'
       Get-ChildItem Env: | % { Write-Output "$($_.Key): $($_.Value)"  }
+      # override RUBYOPT so we don't use UTF-8 encoding when running specs
+      # if Ruby is installed via RubyInstaller it probably defaults to '-Eutf-8'
+      $Env:RUBYOPT = ''
       # list current OpenSSL install
       gem list openssl
       ruby -ropenssl -e 'puts \"OpenSSL Version - #{OpenSSL::OPENSSL_VERSION}\"; puts \"OpenSSL Library Version - #{OpenSSL::OPENSSL_LIBRARY_VERSION}\"'