Prevent security warning from github

Author: benoittgt

Gemfile

@@ -26,9 +26,10 @@ elsif RUBY_VERSION < '1.9'
   gem 'ffi', '< 1.9.19' # ffi dropped Ruby 1.8 support in 1.9.19
 elsif RUBY_VERSION < '2.0'
   gem 'ffi', '< 1.11.0' # ffi dropped Ruby 1.9 support in 1.11.0
+else
+  gem 'ffi', '~> 1.9.25'
 end
 
-
 # No need to run rubocop on earlier versions
 if RUBY_VERSION >= '2.4' && RUBY_ENGINE == 'ruby'
   gem "rubocop", "~> 0.52.1"