Avoid failing on Ruby 2.6 on Windows and Appveyor

Also avoid installing ffi 1.11.0 on Ruby 1.9.x

Related: rspec/rspec-core#2651

Author: benoittgt

Gemfile

@@ -20,14 +20,13 @@ if RUBY_VERSION < '2.0.0' || RUBY_ENGINE == 'java'
   gem 'json', '< 2.0.0' # is a dependency of simplecov
 end
 
-if RUBY_VERSION < '2.0.0' && !!(RbConfig::CONFIG['host_os'] =~ /cygwin|mswin|mingw|bccwin|wince|emx/)
-  gem 'ffi', '< 1.9.15' # allow ffi to be installed on older rubies on windows
-elsif RUBY_VERSION < '1.9'
-  gem 'ffi', '< 1.9.19' # ffi dropped Ruby 1.8 support in 1.9.19
+if RUBY_VERSION < '2.2.0' && !!(RbConfig::CONFIG['host_os'] =~ /cygwin|mswin|mingw|bccwin|wince|emx/)
+  gem 'ffi', '< 1.10'
 elsif RUBY_VERSION < '2.0'
-  gem 'ffi', '< 1.11.0' # ffi dropped Ruby 1.9 support in 1.11.0
+  # ffi dropped Ruby 1.8 support in 1.9.19 and Ruby 1.9 support in 1.11.0
+  gem 'ffi', '< 1.9.19'
 else
-  gem 'ffi', '~> 1.9.25'
+  gem 'ffi', '~> 1.11.0'
 end
 
 # No need to run rubocop on earlier versions