Prevent Github complaining about FFI (#2577)

Author: JonRowe

Gemfile

@@ -36,6 +36,8 @@ if RUBY_VERSION < '2.0.0' && !!(RbConfig::CONFIG['host_os'] =~ /cygwin|mswin|min
   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
+else
+  gem 'ffi', '~> 1.9.25'
 end
 
 platforms :jruby do