Use an earlier version of ffi with Ruby 2.3 on Windows.

Bundler 2.3.23 fails to pick up the binary version of 15.5.4 and fails
to compile from source. Use 15.4.2 instead.

Gemfile

@@ -20,6 +20,9 @@ group :test do
   # doesn't get used. The using the source version results in a segmentation
   # fault during libffi initialization.
   #
+  # Binaries of 15.5.0 to 15.5.4 are declared as compatible with Ruby >= 2.3,
+  # but don't get used with Bundler 2.3.23 and Ruby 2.3 on Windows.
+  #
   # Limit to earlier compatible versions.
-  gem 'ffi', '< 1.15.5' if RUBY_VERSION < '2.4' && RUBY_PLATFORM =~ /mingw/
+  gem 'ffi', '< 1.15.0' if RUBY_VERSION < '2.4' && RUBY_PLATFORM =~ /mingw/
 end